nix-config/overlays/qutebrowser-tree-tabs.diff

diff --git a/qutebrowser/browser/browsertab.py b/qutebrowser/browser/browsertab.py
index 47cba592288..5630dc8cf18 100644
--- a/qutebrowser/browser/browsertab.py
+++ b/qutebrowser/browser/browsertab.py
@@ -52,6 +52,8 @@
     from qutebrowser.browser import webelem
     from qutebrowser.browser.inspector import AbstractWebInspector

+from qutebrowser.mainwindow.treetabwidget import TreeTabWidget
+from qutebrowser.misc.notree import Node

 tab_id_gen = itertools.count(0)
 _WidgetType = Union["QWebView", "QWebEngineView"]
@@ -1058,6 +1060,11 @@ def __init__(self, *, win_id: int,
             self, parent=self)
         self.backend: Optional[usertypes.Backend] = None

+        if parent and isinstance(parent, TreeTabWidget):
+            self.node: AbstractTab = Node(self, parent=parent.tree_root)
+        else:
+            self.node: AbstractTab = Node(self, parent=None)
+
         # If true, this tab has been requested to be removed (or is removed).
         self.pending_removal = False
         self.shutting_down.connect(functools.partial(
diff --git a/qutebrowser/browser/commands.py b/qutebrowser/browser/commands.py
index 3b38c44c05b..d1a1cf6be69 100644
--- a/qutebrowser/browser/commands.py
+++ b/qutebrowser/browser/commands.py
@@ -20,6 +20,7 @@
 import os.path
 import shlex
 import functools
+import urllib.parse
 from typing import cast, Callable, Dict, Union, Optional

 from qutebrowser.qt.widgets import QApplication, QTabBar
@@ -34,7 +35,7 @@
 from qutebrowser.utils import (message, usertypes, log, qtutils, urlutils,
                                objreg, utils, standarddir, debug)
 from qutebrowser.utils.usertypes import KeyMode
-from qutebrowser.misc import editor, guiprocess, objects
+from qutebrowser.misc import editor, guiprocess, objects, notree
 from qutebrowser.completion.models import urlmodel, miscmodels
 from qutebrowser.mainwindow import mainwindow, windowundo

@@ -112,6 +113,7 @@ def _open(
         background: bool = False,
         window: bool = False,
         related: bool = False,
+        sibling: bool = False,
         private: Optional[bool] = None,
     ) -> None:
         """Helper function to open a page.
@@ -123,6 +125,7 @@ def _open(
             window: Whether to open in a new window
             private: If opening a new window, open it in private browsing mode.
                      If not given, inherit the current window's mode.
+            sibling: Open tab in a sibling node of the currently focused tab.
         """
         urlutils.raise_cmdexc_if_invalid(url)
         tabbed_browser = self._tabbed_browser
@@ -135,10 +138,16 @@ def _open(
             tabbed_browser = self._new_tabbed_browser(private)
             tabbed_browser.tabopen(url)
             tabbed_browser.window().show()
-        elif tab:
-            tabbed_browser.tabopen(url, background=False, related=related)
-        elif background:
-            tabbed_browser.tabopen(url, background=True, related=related)
+        elif tab or background:
+            if tabbed_browser.is_treetabbedbrowser:
+                tabbed_browser.tabopen(url, background=background,
+                                       related=related, sibling=sibling)
+            elif sibling:
+                raise cmdutils.CommandError("--sibling flag only works with \
+                                            tree-tab enabled")
+            else:
+                tabbed_browser.tabopen(url, background=background,
+                                       related=related)
         else:
             widget = self._current_widget()
             widget.load_url(url)
@@ -220,7 +229,8 @@ def _get_selection_override(self, prev, next_, opposite):
                                  "{!r}!".format(conf_selection))
         return None

-    def _tab_close(self, tab, prev=False, next_=False, opposite=False):
+    def _tab_close(self, tab, prev=False, next_=False,
+                   opposite=False, new_undo=True):
         """Helper function for tab_close be able to handle message.async.

         Args:
@@ -236,17 +246,17 @@ def _tab_close(self, tab, prev=False, next_=False, opposite=False):
                                                           opposite)

         if selection_override is None:
-            self._tabbed_browser.close_tab(tab)
+            self._tabbed_browser.close_tab(tab, new_undo=new_undo)
         else:
             old_selection_behavior = tabbar.selectionBehaviorOnRemove()
             tabbar.setSelectionBehaviorOnRemove(selection_override)
-            self._tabbed_browser.close_tab(tab)
+            self._tabbed_browser.close_tab(tab, new_undo=new_undo)
             tabbar.setSelectionBehaviorOnRemove(old_selection_behavior)

     @cmdutils.register(instance='command-dispatcher', scope='window')
     @cmdutils.argument('count', value=cmdutils.Value.count)
     def tab_close(self, prev=False, next_=False, opposite=False,
-                  force=False, count=None):
+                  force=False, count=None, recursive=False):
         """Close the current/[count]th tab.

         Args:
@@ -255,15 +265,37 @@ def tab_close(self, prev=False, next_=False, opposite=False,
             opposite: Force selecting the tab in the opposite direction of
                       what's configured in 'tabs.select_on_remove'.
             force: Avoid confirmation for pinned tabs.
+            recursive: Close all descendents (tree-tabs) as well as current tab
             count: The tab index to close, or None
         """
         tab = self._cntwidget(count)
+        tabbed_browser = self._tabbed_browser
         if tab is None:
             return
-        close = functools.partial(self._tab_close, tab, prev,
-                                  next_, opposite)
-
-        self._tabbed_browser.tab_close_prompt_if_pinned(tab, force, close)
+        if (tabbed_browser.is_treetabbedbrowser and recursive and not
+                tab.node.collapsed):
+            # if collapsed, recursive is the same as normal close
+            new_undo = True  # only for first one
+            for descendent in tab.node.traverse(notree.TraverseOrder.POST_R,
+                                                True):
+                if self._tabbed_browser.widget.indexOf(descendent.value) > -1:
+                    close = functools.partial(self._tab_close,
+                                              descendent.value, prev, next_,
+                                              opposite, new_undo)
+                    tabbed_browser.tab_close_prompt_if_pinned(tab, force,
+                                                              close)
+                    new_undo = False
+                else:
+                    tab = descendent.value
+                    tab.private_api.shutdown()
+                    tab.deleteLater()
+                    tab.layout().unwrap()
+        else:
+            # this also applied to closing collapsed tabs
+            # logic for that is in TreeTabbedBrowser
+            close = functools.partial(self._tab_close, tab, prev,
+                                      next_, opposite)
+            tabbed_browser.tab_close_prompt_if_pinned(tab, force, close)

     @cmdutils.register(instance='command-dispatcher', scope='window',
                        name='tab-pin')
@@ -288,8 +320,9 @@ def tab_pin(self, count=None):
     @cmdutils.register(instance='command-dispatcher', name='open',
                        maxsplit=0, scope='window')
     @cmdutils.argument('url', completion=urlmodel.url)
+    @cmdutils.argument('sibling', flag='S')
     @cmdutils.argument('count', value=cmdutils.Value.count)
-    def openurl(self, url=None, related=False,
+    def openurl(self, url=None, related=False, sibling=False,
                 bg=False, tab=False, window=False, count=None, secure=False,
                 private=False):
         """Open a URL in the current/[count]th tab.
@@ -303,6 +336,8 @@ def openurl(self, url=None, related=False,
             window: Open in a new window.
             related: If opening a new tab, position the tab as related to the
                      current one (like clicking on a link).
+            sibling: If opening a new tab, position the as a sibling of the
+                     current one.
             count: The tab index to open the URL in, or None.
             secure: Force HTTPS.
             private: Open a new window in private browsing mode.
@@ -321,8 +356,8 @@ def openurl(self, url=None, related=False,
                 bg = True

             if tab or bg or window or private:
-                self._open(cur_url, tab, bg, window, related=related,
-                           private=private)
+                self._open(cur_url, tab, bg, window, private=private,
+                           related=related, sibling=sibling)
             else:
                 curtab = self._cntwidget(count)
                 if curtab is None:
@@ -461,11 +496,40 @@ def tab_take(self, index, keep=False):
         if not keep:
             tabbed_browser.close_tab(tab, add_undo=False, transfer=True)

+    def _tree_tab_give(self, tabbed_browser, keep):
+        """Helper function to simplify tab-give."""
+        uid_map = {1: 1}
+        traversed = list(self._current_widget().node.traverse())
+        # first pass: open tabs
+        for node in traversed:
+            tab = tabbed_browser.tabopen(node.value.url())
+
+            uid_map[node.uid] = tab.node.uid
+
+        # second pass: copy tree structure over
+        newroot = tabbed_browser.widget.tree_root
+        oldroot = self._tabbed_browser.widget.tree_root
+        for node in traversed:
+            if node.parent is not oldroot:
+                uid = uid_map[node.uid]
+                new_node = newroot.get_descendent_by_uid(uid)
+                parent_uid = uid_map[node.parent.uid]
+                new_parent = newroot.get_descendent_by_uid(parent_uid)
+                new_node.parent = new_parent
+
+        # third pass: remove tabs from old window
+        if not keep:
+            for _node in traversed:
+                self._tabbed_browser.close_tab(self._current_widget(),
+                                               add_undo=False,
+                                               transfer=True)
+
     @cmdutils.register(instance='command-dispatcher', scope='window')
     @cmdutils.argument('win_id', completion=miscmodels.window)
     @cmdutils.argument('count', value=cmdutils.Value.count)
     def tab_give(self, win_id: int = None, keep: bool = False,
-                 count: int = None, private: bool = False) -> None:
+                 count: int = None, private: bool = False,
+                 recursive: bool = False) -> None:
         """Give the current tab to a new or existing window if win_id given.

         If no win_id is given, the tab will get detached into a new window.
@@ -474,6 +538,7 @@ def tab_give(self, win_id: int = None, keep: bool = False,
             win_id: The window ID of the window to give the current tab to.
             keep: If given, keep the old tab around.
             count: Overrides win_id (index starts at 1 for win_id=0).
+            recursive: Whether to move the entire subtree starting at the tab.
             private: If the tab should be detached into a private instance.
         """
         if config.val.tabs.tabs_are_windows:
@@ -505,13 +570,15 @@ def tab_give(self, win_id: int = None, keep: bool = False,
                 raise cmdutils.CommandError(
                     "The window with id {} is not private".format(win_id))

-        tabbed_browser.tabopen(self._current_url())
-        tabbed_browser.window().show()
-
-        if not keep:
-            self._tabbed_browser.close_tab(self._current_widget(),
-                                           add_undo=False,
-                                           transfer=True)
+        if recursive and tabbed_browser.is_treetabbedbrowser:
+            self._tree_tab_give(tabbed_browser, keep)
+        else:
+            tabbed_browser.tabopen(self._current_url())
+            tabbed_browser.window().show()
+            if not keep:
+                self._tabbed_browser.close_tab(self._current_widget(),
+                                               add_undo=False,
+                                               transfer=True)

     def _back_forward(
         self, *,
@@ -889,43 +956,79 @@ def undo(self, window: bool = False,

     @cmdutils.register(instance='command-dispatcher', scope='window')
     @cmdutils.argument('count', value=cmdutils.Value.count)
-    def tab_prev(self, count=1):
+    def tab_prev(self, count=1, sibling=False):
         """Switch to the previous tab, or switch [count] tabs back.

         Args:
             count: How many tabs to switch back.
+            sibling: Whether to focus the previous tree sibling.
         """
         if self._count() == 0:
             # Running :tab-prev after last tab was closed
             # See https://github.com/qutebrowser/qutebrowser/issues/1448
             return
-        newidx = self._current_index() - count
-        if newidx >= 0:
-            self._set_current_index(newidx)
-        elif config.val.tabs.wrap:
-            self._set_current_index(newidx % self._count())
+        if sibling and self._tabbed_browser.is_treetabbedbrowser:
+            cur_node = self._current_widget().node
+            siblings = list(cur_node.parent.children)
+
+            if siblings and len(siblings) > 1:
+                node_idx = siblings.index(cur_node)
+                new_idx = node_idx - count
+                if new_idx >= 0 or config.val.tabs.wrap:
+                    target_node = siblings[(node_idx-count) % len(siblings)]
+                    idx = self._tabbed_browser.widget.indexOf(
+                        target_node.value)
+                    self._set_current_index(idx)
+                else:
+                    log.webview.debug("First sibling")
+            else:
+                log.webview.debug("No siblings")
         else:
-            log.webview.debug("First tab")
+            newidx = self._current_index() - count
+            if newidx >= 0:
+                self._set_current_index(newidx)
+            elif config.val.tabs.wrap:
+                self._set_current_index(newidx % self._count())
+            else:
+                log.webview.debug("First tab")

     @cmdutils.register(instance='command-dispatcher', scope='window')
     @cmdutils.argument('count', value=cmdutils.Value.count)
-    def tab_next(self, count=1):
+    def tab_next(self, count=1, sibling=False):
         """Switch to the next tab, or switch [count] tabs forward.

         Args:
             count: How many tabs to switch forward.
+            sibling: Whether to focus the next tree sibling.
         """
         if self._count() == 0:
             # Running :tab-next after last tab was closed
             # See https://github.com/qutebrowser/qutebrowser/issues/1448
             return
-        newidx = self._current_index() + count
-        if newidx < self._count():
-            self._set_current_index(newidx)
-        elif config.val.tabs.wrap:
-            self._set_current_index(newidx % self._count())
+        if sibling and self._tabbed_browser.is_treetabbedbrowser:
+            cur_node = self._current_widget().node
+            siblings = list(cur_node.parent.children)
+
+            if siblings and len(siblings) > 1:
+                node_idx = siblings.index(cur_node)
+                new_idx = node_idx + count
+                if new_idx < len(siblings) or config.val.tabs.wrap:
+                    target_node = siblings[new_idx % len(siblings)]
+                    idx = self._tabbed_browser.widget.indexOf(
+                        target_node.value)
+                    self._set_current_index(idx)
+                else:
+                    log.webview.debug("Last sibling")
+            else:
+                log.webview.debug("No siblings")
         else:
-            log.webview.debug("Last tab")
+            newidx = self._current_index() + count
+            if newidx < self._count():
+                self._set_current_index(newidx)
+            elif config.val.tabs.wrap:
+                self._set_current_index(newidx % self._count())
+            else:
+                log.webview.debug("Last tab")

     def _resolve_tab_index(self, index):
         """Resolve a tab index to the tabbedbrowser and tab.
@@ -1005,7 +1108,8 @@ def tab_select(self, index=None, count=None):
         tabbed_browser.widget.setCurrentWidget(tab)

     @cmdutils.register(instance='command-dispatcher', scope='window')
-    @cmdutils.argument('index', choices=['last', 'stack-next', 'stack-prev'],
+    @cmdutils.argument('index', choices=['last', 'parent',
+                                         'stack-next', 'stack-prev'],
                        completion=miscmodels.tab_focus)
     @cmdutils.argument('count', value=cmdutils.Value.count)
     def tab_focus(self, index: Union[str, int] = None,
@@ -1016,11 +1120,15 @@ def tab_focus(self, index: Union[str, int] = None,
         If both are given, use count.

         Args:
-            index: The tab index to focus, starting with 1. The special value
-                   `last` focuses the last focused tab (regardless of count),
-                   and `stack-prev`/`stack-next` traverse a stack of visited
-                   tabs. Negative indices count from the end, such that -1 is
-                   the last tab.
+            index: The tab index to focus, starting with 1. Negative indices
+                   count from the end, such that -1 is the last tab. Special
+                   values are:
+                       - `last` focuses the last focused tab (regardless of
+                         count).
+                       - `parent` focuses the parent tab in the tree hierarchy,
+                         if `tabs.tree_tabs` is enabled.
+                       - `stack-prev`/`stack-next` traverse a stack of visited
+                         tabs.
             count: The tab index to focus, starting with 1.
             no_last: Whether to avoid focusing last tab if already focused.
         """
@@ -1030,6 +1138,22 @@ def tab_focus(self, index: Union[str, int] = None,
             assert isinstance(index, str)
             self._tab_focus_stack(index)
             return
+        elif index == 'parent' and self._tabbed_browser.is_treetabbedbrowser:
+            node = self._current_widget().node
+            path = node.path
+            if count:
+                if count < len(path):
+                    path_idx = 0 - count - 1  # path[-1] is node, so shift by 1
+                else:
+                    path_idx = 1  # first non-root node
+            else:
+                path_idx = -2  # immediate parent (path[-1] is node)
+
+            target_node = path[path_idx]
+            if node is target_node or target_node.value is None:
+                raise cmdutils.CommandError("Tab has no parent! ")
+            target_tab = target_node.value
+            index = self._tabbed_browser.widget.indexOf(target_tab) + 1
         elif index is None:
             message.warning(
                 "Using :tab-focus without count is deprecated, "
@@ -1069,17 +1193,31 @@ def tab_move(self, index: Union[str, int] = None, count: int = None) -> None:
                    If moving absolutely: New position (default: 0). This
                    overrides the index argument, if given.
         """
+        # pylint: disable=invalid-unary-operand-type
+        # https://github.com/PyCQA/pylint/issues/1472
         if index in ["+", "-"]:
             # relative moving
             new_idx = self._current_index()
             delta = 1 if count is None else count
-            if index == "-":
-                new_idx -= delta
-            elif index == "+":  # pragma: no branch
-                new_idx += delta

-            if config.val.tabs.wrap:
-                new_idx %= self._count()
+            if self._tabbed_browser.is_treetabbedbrowser:
+                node = self._current_widget().node
+                parent = node.parent
+                siblings = list(parent.children)
+
+                if len(siblings) <= 1:
+                    return
+                rel_idx = siblings.index(node)
+                rel_idx += delta if index == '+' else - delta
+                rel_idx %= len(siblings)
+                new_idx = self._tabbed_browser.widget.indexOf(
+                    siblings[rel_idx].value)
+
+            else:
+                new_idx += delta if index == '+' else - delta
+
+                if config.val.tabs.wrap:
+                    new_idx %= self._count()
         else:
             # pylint: disable=else-if-used
             # absolute moving
@@ -1102,7 +1240,34 @@ def tab_move(self, index: Union[str, int] = None, count: int = None) -> None:
         cur_idx = self._current_index()
         cmdutils.check_overflow(cur_idx, 'int')
         cmdutils.check_overflow(new_idx, 'int')
-        self._tabbed_browser.widget.tabBar().moveTab(cur_idx, new_idx)
+
+        if self._tabbed_browser.is_treetabbedbrowser:
+            # self._tree_tab_move(new_idx)
+            new_idx += 1  # tree-tabs indexes start at 1 (0 is hidden root tab)
+            tab = self._current_widget()
+
+            # traverse order is the same as display order
+            # so indexing works correctly
+            tree_root = self._tabbed_browser.widget.tree_root
+            tabs = list(tree_root.traverse(render_collapsed=False))
+            target_node = tabs[new_idx]
+            if tab.node in target_node.path:
+                raise cmdutils.CommandError("Can't move tab to a descendent"
+                                            " of itself")
+
+            new_parent = target_node.parent
+            # we need index relative to parent for correct placement
+            dest_tab = tabs[new_idx]
+            new_idx_relative = new_parent.children.index(dest_tab)
+
+            tab.node.parent = None  # avoid duplicate errors
+            siblings = list(new_parent.children)
+            siblings.insert(new_idx_relative, tab.node)
+            new_parent.children = siblings
+
+            self._tabbed_browser.widget.tree_tab_update()
+        else:
+            self._tabbed_browser.widget.tabBar().moveTab(cur_idx, new_idx)

     @cmdutils.register(instance='command-dispatcher', scope='window',
                        maxsplit=0, no_replace_variables=True)
@@ -1899,3 +2064,123 @@ def fullscreen(self, leave=False, enter=False):

         log.misc.debug('state before fullscreen: {}'.format(
             debug.qflags_key(Qt, window.state_before_fullscreen)))
+
+    @cmdutils.register(instance='command-dispatcher', scope='window',
+                       tree_tab=True)
+    @cmdutils.argument('count', value=cmdutils.Value.count)
+    def tree_tab_promote(self, count=1):
+        """Promote a tab so it becomes next sibling of its parent.
+
+        Observes tabs.new_position.tree.promote in positioning the tab among
+        new siblings.
+
+        Args:
+            count: How many levels the tabs should be promoted to
+        """
+        if not self._tabbed_browser.is_treetabbedbrowser:
+            raise cmdutils.CommandError('Tree-tabs are disabled')
+        config_position = config.val.tabs.new_position.tree.promote
+        try:
+            self._current_widget().node.promote(count, config_position)
+        except notree.TreeError:
+            raise cmdutils.CommandError('Tab has no parent!')
+        finally:
+            self._tabbed_browser.widget.tree_tab_update()
+
+    @cmdutils.register(instance='command-dispatcher', scope='window',
+                       tree_tab=True)
+    def tree_tab_demote(self):
+        """Demote a tab making it children of its previous adjacent sibling.
+
+        Observes tabs.new_position.tree.demote in positioning the tab among new
+        siblings.
+        """
+        if not self._tabbed_browser.is_treetabbedbrowser:
+            raise cmdutils.CommandError('Tree-tabs are disabled')
+        cur_node = self._current_widget().node
+
+        config_position = config.val.tabs.new_position.tree.demote
+        try:
+            cur_node.demote(config_position)
+        except notree.TreeError:
+            raise cmdutils.CommandError('Tab has no previous sibling!')
+        finally:
+            self._tabbed_browser.widget.tree_tab_update()
+
+    @cmdutils.register(instance='command-dispatcher', scope='window',
+                       tree_tab=True)
+    @cmdutils.argument('count', value=cmdutils.Value.count)
+    def tree_tab_toggle_hide(self, count=None):
+        """If the current tab's children are shown hide them, and vice-versa.
+
+        This toggles the current tab's node's `collapsed` attribute.
+
+        Args:
+            count: Which tab to collapse
+        """
+        if not self._tabbed_browser.is_treetabbedbrowser:
+            raise cmdutils.CommandError('Tree-tabs are disabled')
+        tab = self._cntwidget(count)
+        if not tab.node.children:
+            return
+        tab.node.collapsed = not tab.node.collapsed
+
+        self._tabbed_browser.widget.tree_tab_update()
+
+    @cmdutils.register(instance='command-dispatcher', scope='window',
+                       tree_tab=True)
+    @cmdutils.argument('count', value=cmdutils.Value.count)
+    def tree_tab_cycle_hide(self, count=1):
+        """Hides levels of descendents: children, grandchildren, and so on.
+
+        Args:
+            count: How many levels to hide.
+        """
+        if not self._tabbed_browser.is_treetabbedbrowser:
+            raise cmdutils.CommandError('Tree-tabs are disabled')
+        while count > 0:
+            tab = self._current_widget()
+            self._tabbed_browser.cycle_hide_tab(tab.node)
+            count -= 1
+
+        self._tabbed_browser.widget.tree_tab_update()
+
+    @cmdutils.register(instance='command-dispatcher', scope='window',
+                       tree_tab=True)
+    def tree_tab_create_group(self, *name, related=False,
+                              background=False):
+        """Wrapper around :open qute://treegroup/name. Correctly escapes names.
+
+        Example: `:tree-tab-create-group Foo Bar` calls
+            `:open qute://treegroup/Foo%20Bar`
+
+        Args:
+            name: Name of the group to create
+            related: whether to open as a child of current tab or under root
+            background: whether to open in a background tab
+        """
+        title = ' '.join(name)
+        path = urllib.parse.quote(title)
+        if background:
+            self.openurl('qute://treegroup/' + path, related=related, bg=True)
+        else:
+            self.openurl('qute://treegroup/' + path, related=related, tab=True)
+
+    @cmdutils.register(instance='command-dispatcher', scope='window',
+                       tree_tab=True)
+    @cmdutils.argument('count', value=cmdutils.Value.count)
+    def tree_tab_suspend_children(self, count=None):
+        """Suspends all descendent of a tab to reduce memory usage.
+
+        Args:
+            count: Target tab.
+        """
+        tab = self._cntwidget(count)
+        for descendent in tab.node.traverse():
+            cur_tab = descendent.value
+            if cur_tab and cur_tab is not tab:
+                cur_url = cur_tab.url().url()
+                if not cur_url.startswith("qute://"):
+                    new_url = self._parse_url(
+                        "qute://back/#" + cur_tab.title())
+                    cur_tab.load_url(new_url)
diff --git a/qutebrowser/browser/qutescheme.py b/qutebrowser/browser/qutescheme.py
index 25834670b11..15e4bc769e4 100644
--- a/qutebrowser/browser/qutescheme.py
+++ b/qutebrowser/browser/qutescheme.py
@@ -588,6 +588,18 @@ def qute_warning(url: QUrl) -> _HandlerRet:
     return 'text/html', src


+@add_handler('treegroup')
+def qute_treegroup(url):
+    """Handler for qute://treegroup/x.
+
+    Makes an empty tab with a title, for use with tree-tabs as a grouping
+    feature.
+    """
+    src = jinja.render('tree_group.html',
+                       title=url.path()[1:])
+    return 'text/html', src
+
+
 @add_handler('resource')
 def qute_resource(url: QUrl) -> _HandlerRet:
     """Handler for qute://resource."""
diff --git a/qutebrowser/commands/command.py b/qutebrowser/commands/command.py
index 655a5a336ad..76a44934e97 100644
--- a/qutebrowser/commands/command.py
+++ b/qutebrowser/commands/command.py
@@ -65,6 +65,7 @@ class Command:
                  both)
         no_replace_variables: Don't replace variables like {url}
         modes: The modes the command can be executed in.
+        tree_tab: Whether the command is a tree-tabs command
         _qute_args: The saved data from @cmdutils.argument
         _count: The count set for the command.
         _instance: The object to bind 'self' to.
@@ -78,7 +79,7 @@ class Command:
     def __init__(self, *, handler, name, instance=None, maxsplit=None,
                  modes=None, not_modes=None, debug=False, deprecated=False,
                  no_cmd_split=False, star_args_optional=False, scope='global',
-                 backend=None, no_replace_variables=False):
+                 backend=None, no_replace_variables=False, tree_tab=False):
         if modes is not None and not_modes is not None:
             raise ValueError("Only modes or not_modes can be given!")
         if modes is not None:
@@ -107,6 +108,7 @@ def __init__(self, *, handler, name, instance=None, maxsplit=None,
         self.handler = handler
         self.no_cmd_split = no_cmd_split
         self.backend = backend
+        self.tree_tab = tree_tab
         self.no_replace_variables = no_replace_variables

         self.docparser = docutils.DocstringParser(handler)
diff --git a/qutebrowser/completion/models/util.py b/qutebrowser/completion/models/util.py
index 69a192f686e..16452d7b571 100644
--- a/qutebrowser/completion/models/util.py
+++ b/qutebrowser/completion/models/util.py
@@ -21,6 +21,7 @@

 from qutebrowser.utils import usertypes
 from qutebrowser.misc import objects
+from qutebrowser.config import config


 DeleteFuncType = Callable[[Sequence[str]], None]
@@ -44,8 +45,9 @@ def get_cmd_completions(info, include_hidden, include_aliases, prefix=''):
         hide_debug = obj.debug and not objects.args.debug
         hide_mode = (usertypes.KeyMode.normal not in obj.modes and
                      not include_hidden)
+        hide_tree = obj.tree_tab and not config.cache['tabs.tree_tabs']
         hide_ni = obj.name == 'Ni!'
-        if not (hide_debug or hide_mode or obj.deprecated or hide_ni):
+        if not (hide_tree or hide_debug or hide_mode or obj.deprecated or hide_ni):
             bindings = ', '.join(cmd_to_keys.get(obj.name, []))
             cmdlist.append((prefix + obj.name, obj.desc, bindings))

diff --git a/qutebrowser/config/configdata.yml b/qutebrowser/config/configdata.yml
index 0b9d669dceb..05526260e02 100644
--- a/qutebrowser/config/configdata.yml
+++ b/qutebrowser/config/configdata.yml
@@ -2224,6 +2224,40 @@ tabs.new_position.stacking:
     Only applies for `next` and `prev` values of `tabs.new_position.related`
     and `tabs.new_position.unrelated`.

+tabs.new_position.tree.new_child:
+  default: first
+  type: NewChildPosition
+  desc: >-
+    Position of new children among siblings, e.g. after calling `:open
+    --relative ...` or following a link.
+
+tabs.new_position.tree.new_sibling:
+  default: first
+  type: NewTabPosition
+  desc: >-
+    Position of siblings, e.g. after calling `:open --sibling ...`.
+
+tabs.new_position.tree.new_toplevel:
+  default: last
+  type: NewTabPosition
+  desc: >-
+    Position of new top-level tabs related to the topmost ancestor of current
+    tab, e.g. when calling `:open ...` without `--relative` or `--sibling`.
+
+tabs.new_position.tree.promote:
+  default: next
+  type: NewTabPosition
+  desc: >-
+    Position at which a tab is placed among its new siblings after being
+    promoted with `:tree-tab-promote`
+
+tabs.new_position.tree.demote:
+  default: last
+  type: NewChildPosition
+  desc: >-
+    Position at which a tab is placed among its new siblings after being
+    demoted with `:tree-tab-demote`
+
 tabs.padding:
   default:
     top: 0
@@ -2289,7 +2323,7 @@ tabs.title.elide:
   desc: Position of ellipsis in truncated title of tabs.

 tabs.title.format:
-  default: '{audio}{index}: {current_title}'
+  default: '{tree}{collapsed}{audio}{index}: {current_title}'
   type:
     name: FormatString
     fields:
@@ -2307,12 +2341,16 @@ tabs.title.format:
       - current_url
       - protocol
       - audio
+      - collapsed
+      - tree
     none_ok: true
   desc: |
     Format to use for the tab title.
     The following placeholders are defined:

     * `{perc}`: Percentage as a string like `[10%]`.
+    * `{collapsed}`: If children tabs are hidden, the string `[...]`, empty otherwise
+    * `{tree}`: The ASCII tree prefix of current tab.
     * `{perc_raw}`: Raw percentage, e.g. `10`.
     * `{current_title}`: Title of the current web page.
     * `{title_sep}`: The string `" - "` if a title is set, empty otherwise.
@@ -2348,6 +2386,8 @@ tabs.title.format_pinned:
       - current_url
       - protocol
       - audio
+      - collapsed
+      - tree
     none_ok: true
   desc: Format to use for the tab title for pinned tabs. The same placeholders
     like for `tabs.title.format` are defined.
@@ -2446,6 +2486,12 @@ tabs.wrap:
   type: Bool
   desc: Wrap when changing tabs.

+tabs.tree_tabs:
+  default: false
+  type: Bool
+  desc: Enable tree-tabs mode.
+  restart: true
+
 tabs.focus_stack_size:
   default: 10
   type:
@@ -3831,6 +3877,17 @@ bindings.default:
         all no-3rdparty never ;; reload
       tCu: config-cycle -p -u {url} content.cookies.accept
         all no-3rdparty never ;; reload
+      zH: tree-tab-promote
+      zL: tree-tab-demote
+      zJ: tab-next -s
+      zK: tab-prev -s
+      zd: tab-close -r
+      zg: set-cmd-text -s :tree-tab-create-group -r
+      zG: set-cmd-text -s :tree-tab-create-group
+      za: tree-tab-toggle-hide
+      zp: tab-focus parent
+      zo: set-cmd-text --space :open -tr
+      zO: set-cmd-text --space :open -tS
     insert:
       <Ctrl-E>: edit-text
       <Shift-Ins>: insert-text -- {primary}
diff --git a/qutebrowser/config/configtypes.py b/qutebrowser/config/configtypes.py
index b451744c338..a10f4aa3bb5 100644
--- a/qutebrowser/config/configtypes.py
+++ b/qutebrowser/config/configtypes.py
@@ -1955,6 +1955,21 @@ def __init__(
             ('last', "At the end."))


+class NewChildPosition(String):
+
+    """How new children are positioned."""
+
+    def __init__(
+        self, *,
+        none_ok: bool = False,
+        completions: _Completions = None,
+    ) -> None:
+        super().__init__(none_ok=none_ok, completions=completions)
+        self.valid_values = ValidValues(
+            ('first', "At the beginning."),
+            ('last', "At the end."))
+
+
 class LogLevel(String):

     """A logging level."""
diff --git a/qutebrowser/html/tree_group.html b/qutebrowser/html/tree_group.html
new file mode 100644
index 00000000000..b3717e52b6e
--- /dev/null
+++ b/qutebrowser/html/tree_group.html
@@ -0,0 +1,65 @@
+{% extends "base.html" %}
+{% block style %}
+h1, p {
+    margin-left: 3rem;
+}
+
+pre {
+    margin-left: 6em;
+}
+{% endblock %}
+{% block content %}
+<h1>
+    {{ title }}
+</h1>
+<p>
+    <em>Group for tree tabs</em>
+</p>
+<pre>
+{% raw %}
+                       _.
+                _~.:'^%^ >@~.
+             ,-~              ? =*=
+           $^_`  `  , '   ,    +   -.,
+        (*-^. , *            ;'       >
+      >.      ,>    .     ' .,.,.      %-.,_ ,.-,
+      #    ' ` " - "     *    .,.      *   .^    `
+     *@!    ., *    ' '    ,    ;'   '         .  %!
+      &       "  .`      :'              `   '   . `~,
+     &    '        .`  '   '  .     '":   :          +.
+     ^      .",  ,       `      '   `   * , '   `      |
+      ]     *   .   , ""]   ..    ` . , `  , "  . . '  ,;,
+      %  '         ::,  ,   /    ,            '   ,     ;
+    .* ,*    /       *%     \  .  .  *'  `    ,    '     '.
+   ?     > .   ,      ::. :;^^.     %`      '        `     @
+  /   '   `/      `    &#@%^^      `&``   `     %;;       %
+ ;:       :%   *  *  :$%)\      '  `@%$        @%^      ,).
+ .    #    %&^     (!*^ .\,.   `      ^@%^  $#%%^  `   >
+ \           :#$%  #^&#   :   `  *      %###$%@!       &
+  |  '  *     %$#@)$*}]           `     `#@#%%^      *^
+   :     *'  *  @%&&^:$   `  '   `%%.  #$$$^^-,     7
+    &;            @#$~~   '   `     @#$%&      $,*.-
+     *...*^  .._   %$$#@!   @ .,  *&&#@
+         :..^   -   !%&@}{#&     @#$@%
+                 --_..%#%%$#&% #&%$#;:
+                        $%#^%#@@%%*&;;
+                         a%##@%%@% ;:;
+                           %####j#:::;
+                            &#%Rj;%;;:
+                            &#%%#;::;
+                            $#%##:%::
+                            "#%%#;:;
+                           ."$###:::
+                           #&$%%#:;:
+                           %&#%%#::;
+                           %&%###;::
+                           &&#%%#:;;
+                          *@&#%#};:;
+                          $#%#%%^:::
+                         *@#$#%#;::;:
+                        %%@#$####@$:;:
+                    ...%###pinusc@$%%:._____
+{% endraw %}
+
+</pre>
+{% endblock %}
diff --git a/qutebrowser/mainwindow/mainwindow.py b/qutebrowser/mainwindow/mainwindow.py
index 5e34a6649fe..405e9fd285b 100644
--- a/qutebrowser/mainwindow/mainwindow.py
+++ b/qutebrowser/mainwindow/mainwindow.py
@@ -21,7 +21,7 @@
 import base64
 import itertools
 import functools
-from typing import List, MutableSequence, Optional, Tuple, cast
+from typing import List, MutableSequence, Optional, Tuple, cast, Union

 from qutebrowser.qt import machinery
 from qutebrowser.qt.core import (pyqtBoundSignal, pyqtSlot, QRect, QPoint, QTimer, Qt,
@@ -208,7 +208,7 @@ def __init__(self, *,
         super().__init__(parent)
         # Late import to avoid a circular dependency
         # - browsertab -> hints -> webelem -> mainwindow -> bar -> browsertab
-        from qutebrowser.mainwindow import tabbedbrowser
+        from qutebrowser.mainwindow import treetabbedbrowser, tabbedbrowser
         from qutebrowser.mainwindow.statusbar import bar

         self.setAttribute(Qt.WidgetAttribute.WA_DeleteOnClose)
@@ -237,8 +237,14 @@ def __init__(self, *,

         self.is_private = config.val.content.private_browsing or private

-        self.tabbed_browser: tabbedbrowser.TabbedBrowser = tabbedbrowser.TabbedBrowser(
-            win_id=self.win_id, private=self.is_private, parent=self)
+        self.tabbed_browser: Union[tabbedbrowser.TabbedBrowser,
+                                   treetabbedbrowser.TreeTabbedBrowser]
+        if config.val.tabs.tree_tabs:
+            self.tabbed_browser = treetabbedbrowser.TreeTabbedBrowser(
+                win_id=self.win_id, private=self.is_private, parent=self)
+        else:
+            self.tabbed_browser = tabbedbrowser.TabbedBrowser(
+                win_id=self.win_id, private=self.is_private, parent=self)
         objreg.register('tabbed-browser', self.tabbed_browser, scope='window',
                         window=self.win_id)
         self._init_command_dispatcher()
@@ -510,8 +516,10 @@ def _connect_signals(self):
         mode_manager.keystring_updated.connect(
             self.status.keystring.on_keystring_updated)
         self.status.cmd.got_cmd[str].connect(self._commandrunner.run_safely)
-        self.status.cmd.got_cmd[str, int].connect(self._commandrunner.run_safely)
-        self.status.cmd.returnPressed.connect(self.tabbed_browser.on_cmd_return_pressed)
+        self.status.cmd.got_cmd[str, int].connect(
+            self._commandrunner.run_safely)
+        self.status.cmd.returnPressed.connect(
+            self.tabbed_browser.on_cmd_return_pressed)
         self.status.cmd.got_search.connect(self._command_dispatcher.search)

         # key hint popup
diff --git a/qutebrowser/mainwindow/tabbedbrowser.py b/qutebrowser/mainwindow/tabbedbrowser.py
index da3392a7e81..61f75c2b5a1 100644
--- a/qutebrowser/mainwindow/tabbedbrowser.py
+++ b/qutebrowser/mainwindow/tabbedbrowser.py
@@ -22,8 +22,8 @@
 import weakref
 import datetime
 import dataclasses
-from typing import (
-    Any, Deque, List, Mapping, MutableMapping, MutableSequence, Optional, Tuple)
+from typing import (Any, Deque, List, Mapping,
+                    MutableMapping, MutableSequence, Optional, Tuple)

 from qutebrowser.qt.widgets import QSizePolicy, QWidget, QApplication
 from qutebrowser.qt.core import pyqtSignal, pyqtSlot, QTimer, QUrl, QPoint
@@ -207,6 +207,7 @@ class TabbedBrowser(QWidget):
     resized = pyqtSignal('QRect')
     current_tab_changed = pyqtSignal(browsertab.AbstractTab)
     new_tab = pyqtSignal(browsertab.AbstractTab, int)
+    is_treetabbedbrowser = False
     shutting_down = pyqtSignal()

     def __init__(self, *, win_id, private, parent=None):
@@ -485,6 +486,7 @@ def _remove_tab(self, tab, *, add_undo=True, new_undo=True, crashed=False):
             crashed: Whether we're closing a tab with crashed renderer process.
         """
         idx = self.widget.indexOf(tab)
+
         if idx == -1:
             if crashed:
                 return
@@ -553,17 +555,26 @@ def undo(self, depth=1):
         entries = self.undo_stack[-depth]
         del self.undo_stack[-depth]

+        # we return the tab list because tree_tabs needs it in post_processing
+        new_tabs = []
         for entry in reversed(entries):
             if use_current_tab:
                 newtab = self._tab_by_idx(0)
                 assert newtab is not None
                 use_current_tab = False
             else:
-                newtab = self.tabopen(background=False, idx=entry.index)
+                # FIXME:typing mypy thinks this is None due to @pyqtSlot
+                newtab = self.tabopen(
+                    background=False,
+                    related=False,
+                    idx=entry.index
+                )

             newtab.history.private_api.deserialize(entry.history)
             newtab.set_pinned(entry.pinned)
+            new_tabs.append(newtab)
             newtab.setFocus()
+        return new_tabs

     @pyqtSlot('QUrl', bool)
     def load_url(self, url, newtab):
@@ -651,7 +662,7 @@ def tabopen(

         if idx is None:
             idx = self._get_new_tab_idx(related)
-        self.widget.insertTab(idx, tab, "")
+        idx = self.widget.insertTab(idx, tab, "")

         if url is not None:
             tab.load_url(url)
diff --git a/qutebrowser/mainwindow/tabwidget.py b/qutebrowser/mainwindow/tabwidget.py
index fe9ce1e0676..f2cb1765828 100644
--- a/qutebrowser/mainwindow/tabwidget.py
+++ b/qutebrowser/mainwindow/tabwidget.py
@@ -26,7 +26,7 @@
                           QTimer, QUrl)
 from qutebrowser.qt.widgets import (QTabWidget, QTabBar, QSizePolicy, QProxyStyle,
                              QStyle, QStylePainter, QStyleOptionTab,
-                             QCommonStyle)
+                             QCommonStyle, QWidget)
 from qutebrowser.qt.gui import QIcon, QPalette, QColor

 from qutebrowser.utils import qtutils, objreg, utils, usertypes, log
@@ -36,7 +36,6 @@


 class TabWidget(QTabWidget):
-
     """The tab widget used for TabbedBrowser.

     Signals:
@@ -59,14 +58,14 @@ def __init__(self, win_id, parent=None):
         self.setStyle(TabBarStyle())
         self.setTabBar(bar)
         bar.tabCloseRequested.connect(self.tabCloseRequested)
-        bar.tabMoved.connect(functools.partial(
-            QTimer.singleShot, 0, self.update_tab_titles))
+        bar.tabMoved.connect(self.update_tab_titles)
         bar.currentChanged.connect(self._on_current_changed)
         bar.new_tab_requested.connect(self._on_new_tab_requested)
         self.setSizePolicy(QSizePolicy.Policy.Expanding, QSizePolicy.Policy.Fixed)
         self.setDocumentMode(True)
         self.setUsesScrollButtons(True)
         bar.setDrawBase(False)
+
         self._init_config()
         config.instance.changed.connect(self._init_config)

@@ -92,8 +91,9 @@ def tab_bar(self) -> "TabBar":
         assert isinstance(bar, TabBar), bar
         return bar

-    def _tab_by_idx(self, idx: int) -> Optional[browsertab.AbstractTab]:
+    def _tab_by_idx(self, idx: int) -> Optional[QWidget]:
         """Get the tab at the given index."""
+
         tab = self.widget(idx)
         if tab is not None:
             assert isinstance(tab, browsertab.AbstractTab), tab
@@ -189,6 +189,9 @@ def get_tab_fields(self, idx):
         fields['perc_raw'] = tab.progress()
         fields['backend'] = objects.backend.name
         fields['private'] = ' [Private Mode] ' if tab.is_private else ''
+        fields['tree'] = ''
+        fields['collapsed'] = ''
+
         try:
             if tab.audio.is_muted():
                 fields['audio'] = TabWidget.MUTE_STRING
@@ -347,7 +350,7 @@ def tab_url(self, idx):
         qtutils.ensure_valid(url)
         return url

-    def update_tab_favicon(self, tab: browsertab.AbstractTab) -> None:
+    def update_tab_favicon(self, tab: QWidget) -> None:
         """Update favicon of the given tab."""
         idx = self.indexOf(tab)

diff --git a/qutebrowser/mainwindow/treetabbedbrowser.py b/qutebrowser/mainwindow/treetabbedbrowser.py
new file mode 100644
index 00000000000..c7e10ab180d
--- /dev/null
+++ b/qutebrowser/mainwindow/treetabbedbrowser.py
@@ -0,0 +1,350 @@
+# vim: ft=python fileencoding=utf-8 sts=4 sw=4 et:
+
+# Copyright 2014-2018 Giuseppe Stelluto (pinusc) <giuseppe@gstelluto.com>
+#
+# This file is part of qutebrowser.
+#
+# qutebrowser is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# qutebrowser is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with qutebrowser.  If not, see <https://www.gnu.org/licenses/>.
+
+"""Subclass of TabbedBrowser to provide tree-tab functionality."""
+
+import collections
+import dataclasses
+import datetime
+from typing import List, Dict
+from qutebrowser.qt.widgets import QSizePolicy
+from qutebrowser.qt.core import pyqtSlot, QUrl
+
+from qutebrowser.config import config
+from qutebrowser.mainwindow.tabbedbrowser import TabbedBrowser
+from qutebrowser.mainwindow.treetabwidget import TreeTabWidget
+from qutebrowser.browser import browsertab
+from qutebrowser.misc import notree
+from qutebrowser.utils import log
+
+
+@dataclasses.dataclass
+class _TreeUndoEntry():
+    """Information needed for :undo."""
+
+    url: QUrl
+    history: bytes
+    index: int
+    pinned: bool
+    uid: int
+    parent_node_uid: int
+    children_node_uids: List[int]
+    local_index: int  # index of the tab relative to its siblings
+    created_at: datetime.datetime = dataclasses.field(
+        default_factory=datetime.datetime.now)
+
+    @staticmethod
+    def from_node(node, idx):
+        """Make a TreeUndoEntry from a Node."""
+        url = node.value.url()
+        try:
+            history_data = node.value.history.private_api.serialize()
+        except browsertab.WebTabError:
+            history_data = []
+        pinned = node.value.data.pinned
+        uid = node.uid
+        parent_uid = node.parent.uid
+        children = [n.uid for n in node.children]
+        local_idx = node.index
+        return _TreeUndoEntry(url=url,
+                              history=history_data,
+                              index=idx,
+                              pinned=pinned,
+                              uid=uid,
+                              parent_node_uid=parent_uid,
+                              children_node_uids=children,
+                              local_index=local_idx)
+
+
+class TreeTabbedBrowser(TabbedBrowser):
+    """Subclass of TabbedBrowser to provide tree-tab functionality.
+
+    Extends TabbedBrowser methods (mostly tabopen, undo, and _remove_tab) so
+    that the internal tree is updated after every action.
+
+    Provides methods to hide and show subtrees, and to cycle visibility.
+    """
+
+    is_treetabbedbrowser = True
+
+    def __init__(self, *, win_id, private, parent=None):
+        super().__init__(win_id=win_id, private=private, parent=parent)
+        self.is_treetabbedbrowser = True
+        self.widget = TreeTabWidget(win_id, parent=self)
+        self.widget.tabCloseRequested.connect(self.on_tab_close_requested)
+        self.widget.new_tab_requested.connect(self.tabopen)
+        self.widget.currentChanged.connect(self._on_current_changed)
+        self.cur_fullscreen_requested.connect(self.widget.tabBar().maybe_hide)
+        self.widget.setSizePolicy(QSizePolicy.Policy.Expanding, QSizePolicy.Policy.Expanding)
+        self._reset_stack_counters()
+
+    def _remove_tab(self, tab, *, add_undo=True, new_undo=True, crashed=False):
+        """Handle children positioning after a tab is removed."""
+        node = tab.node
+        # FIXME after the fixme in _add_undo_entry is resolved, no need
+        # to save descendents
+        descendents = tuple(node.traverse(render_collapsed=True))
+
+        super()._remove_tab(tab, add_undo=False, new_undo=False,
+                            crashed=crashed)
+
+        if not tab.url().isEmpty() and tab.url().isValid() and add_undo:
+            idx = self.widget.indexOf(tab)
+            self._add_undo_entry(tab, idx, new_undo)
+
+        parent = node.parent
+
+        if node.collapsed:
+            # node will already be removed from tree
+            # but we need to manually close the tab processes
+            for descendent in descendents:
+                tab = descendent.value
+                tab.private_api.shutdown()
+                tab.deleteLater()
+                tab.layout().unwrap()
+        elif parent:
+            siblings = list(parent.children)
+            children = node.children
+
+            if children:
+                next_node = children[0]
+
+                for n in children[1:]:
+                    n.parent = next_node
+
+                # swap nodes
+                node_idx = siblings.index(node)
+                siblings[node_idx] = next_node
+
+                parent.children = tuple(siblings)
+                node.children = ()
+
+            node.parent = None
+        self.widget.tree_tab_update()
+
+    def _add_undo_entry(self, tab, idx, new_undo):
+        """Save undo entry with tree information.
+
+        This function was removed in tabbedbrowser, but it is still useful here because
+        the mechanism is quite a bit more complex
+        """
+        # TODO see if it's possible to remove duplicate code from
+        # super()._add_undo_entry
+        try:
+            history_data = tab.history.private_api.serialize()
+        except browsertab.WebTabError:
+            pass  # special URL
+        else:
+            node = tab.node
+            uid = node.uid
+            parent_uid = node.parent.uid
+            if not node.collapsed:
+                children = [n.uid for n in node.children]
+                local_idx = node.index
+                entry = _TreeUndoEntry(url=tab.url(),
+                                       history=history_data,
+                                       index=idx,
+                                       pinned=tab.data.pinned,
+                                       uid=uid,
+                                       parent_node_uid=parent_uid,
+                                       children_node_uids=children,
+                                       local_index=local_idx)
+                if new_undo or not self.undo_stack:
+                    self.undo_stack.append([entry])
+                else:
+                    self.undo_stack[-1].append(entry)
+            else:
+                entries = []
+                for descendent in node.traverse(notree.TraverseOrder.POST_R):
+                    entries.append(_TreeUndoEntry.from_node(descendent, 0))
+                    # ensure descendent is not later saved as child as well
+                    descendent.parent = None  # FIXME: Find a way not to change
+                    # the tree
+                if new_undo:
+                    self.undo_stack.append(entries)
+                else:
+                    self.undo_stack[-1] += entries
+
+    def undo(self, depth=1):
+        """Undo removing of a tab or tabs."""
+        # TODO find a way to remove dupe code
+        # probably by getting entries from undo stack, THEN calling super
+        # then post-processing the entries
+
+        # save entries before super().undo() pops them
+        entries = list(self.undo_stack[-depth])
+        new_tabs = super().undo(depth)
+
+        for entry, tab in zip(reversed(entries), new_tabs):
+            if not isinstance(entry, _TreeUndoEntry):
+                continue
+            root = self.widget.tree_root
+            uid = entry.uid
+            parent_uid = entry.parent_node_uid
+            parent_node = root.get_descendent_by_uid(parent_uid)
+            if not parent_node:
+                parent_node = root
+
+            children = []
+            for child_uid in entry.children_node_uids:
+                child_node = root.get_descendent_by_uid(child_uid)
+                children.append(child_node)
+            tab.node.parent = None  # Remove the node from the tree
+            tab.node = notree.Node(tab, parent_node,
+                                   children, uid)
+
+            # correctly reposition the tab
+            local_idx = entry.local_index
+            if tab.node.parent:  # should always be true
+                new_siblings = list(tab.node.parent.children)
+                new_siblings.remove(tab.node)
+                new_siblings.insert(local_idx, tab.node)
+                tab.node.parent.children = new_siblings
+
+        self.widget.tree_tab_update()
+
+    @pyqtSlot('QUrl')
+    @pyqtSlot('QUrl', bool)
+    @pyqtSlot('QUrl', bool, bool)
+    def tabopen(
+            self, url: QUrl = None,
+            background: bool = None,
+            related: bool = True,
+            sibling: bool = False,
+            idx: int = None,
+    ) -> browsertab.AbstractTab:
+        """Open a new tab with a given url.
+
+        Args:
+            related: Whether to set the tab as a child of the currently focused
+                     tab. Follows `tabs.new_position.tree.related`.
+            sibling: Whether to set the tab as a sibling of the currently
+                     focused tab.  Follows `tabs.new_position.tree.sibling`.
+
+        """
+        # pylint: disable=arguments-differ
+        # we save this now because super.tabopen also resets the focus
+        cur_tab = self.widget.currentWidget()
+        tab = super().tabopen(url, background, related, idx)
+
+        tab.node.parent = self.widget.tree_root
+        if cur_tab is None or tab is cur_tab:
+            self.widget.tree_tab_update()
+            return tab
+
+        # get pos
+        if related:
+            pos = config.val.tabs.new_position.tree.new_child
+            parent = cur_tab.node
+            # pos can only be first, last
+        elif sibling:
+            pos = config.val.tabs.new_position.tree.new_sibling
+            parent = cur_tab.node.parent
+            # pos can be first, last, prev, next
+        else:
+            pos = config.val.tabs.new_position.tree.new_toplevel
+            parent = self.widget.tree_root
+
+        self._position_tab(cur_tab, tab, pos, parent, sibling, related, background)
+
+        return tab
+
+    def _position_tab(
+        self,
+        cur_tab: browsertab.AbstractTab,
+        tab: browsertab.AbstractTab,
+        pos: str,
+        parent: notree.Node,
+        sibling: bool = False,
+        related: bool = True,
+        background: bool = None,
+    ) -> None:
+        toplevel = not sibling and not related
+        siblings = list(parent.children)
+        if tab.node in siblings:  # true if parent is tree_root
+            # remove it and add it later in the right position
+            siblings.remove(tab.node)
+
+        if pos == 'first':
+            rel_idx = 0
+            if config.val.tabs.new_position.stacking and related:
+                rel_idx += self._tree_tab_child_rel_idx
+                self._tree_tab_child_rel_idx += 1
+            siblings.insert(rel_idx, tab.node)
+        elif pos in ['prev', 'next'] and (sibling or toplevel):
+            # pivot is the tab relative to which 'prev' or 'next' apply
+            # it is always a member of 'siblings'
+            pivot = cur_tab.node if sibling else cur_tab.node.path[1]
+            direction = -1 if pos == 'prev' else 1
+            rel_idx = 0 if pos == 'prev' else 1
+            tgt_idx = siblings.index(pivot) + rel_idx
+            if config.val.tabs.new_position.stacking:
+                if sibling:
+                    tgt_idx += self._tree_tab_sibling_rel_idx
+                    self._tree_tab_sibling_rel_idx += direction
+                elif toplevel:
+                    tgt_idx += self._tree_tab_toplevel_rel_idx
+                    self._tree_tab_toplevel_rel_idx += direction
+            siblings.insert(tgt_idx, tab.node)
+        else:  # position == 'last'
+            siblings.append(tab.node)
+        parent.children = siblings
+        self.widget.tree_tab_update()
+        if not background:
+            self._reset_stack_counters()
+
+    def _reset_stack_counters(self):
+        self._tree_tab_child_rel_idx = 0
+        self._tree_tab_sibling_rel_idx = 0
+        self._tree_tab_toplevel_rel_idx = 0
+
+    @pyqtSlot(int)
+    def _on_current_changed(self, idx):
+        super()._on_current_changed(idx)
+        self._reset_stack_counters()
+
+    def cycle_hide_tab(self, node):
+        """Utility function for tree_tab_cycle_hide command."""
+        # height = node.height  # height is always rel_height
+        if node.collapsed:
+            node.collapsed = False
+            for descendent in node.traverse(render_collapsed=True):
+                descendent.collapsed = False
+            return
+
+        def rel_depth(n):
+            return n.depth - node.depth
+
+        levels: Dict[int, list] = collections.defaultdict(list)
+        for d in node.traverse(render_collapsed=False):
+            r_depth = rel_depth(d)
+            levels[r_depth].append(d)
+
+        # Remove highest level because it's leaves (or already collapsed)
+        del levels[max(levels.keys())]
+
+        target = 0
+        for level in sorted(levels, reverse=True):
+            nodes = levels[level]
+            if not all(n.collapsed or not n.children for n in nodes):
+                target = level
+                break
+        for n in levels[target]:
+            if not n.collapsed and n.children:
+                n.collapsed = True
diff --git a/qutebrowser/mainwindow/treetabwidget.py b/qutebrowser/mainwindow/treetabwidget.py
new file mode 100644
index 00000000000..12de178c2ec
--- /dev/null
+++ b/qutebrowser/mainwindow/treetabwidget.py
@@ -0,0 +1,105 @@
+# vim: ft=python fileencoding=utf-8 sts=4 sw=4 et:
+
+# Copyright 2014-2018 Giuseppe Stelluto (pinusc) <giuseppe@gstelluto.com>
+#
+# This file is part of qutebrowser.
+#
+# qutebrowser is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# qutebrowser is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with qutebrowser.  If not, see <https://www.gnu.org/licenses/>.
+
+"""Extension of TabWidget for tree-tab functionality."""
+
+from qutebrowser.mainwindow.tabwidget import TabWidget
+from qutebrowser.misc.notree import Node
+from qutebrowser.utils import log
+
+
+class TreeTabWidget(TabWidget):
+    """Tab widget used in TabbedBrowser, with tree-functionality.
+
+    Handles correct rendering of the tree as a tab field, and correct
+    positioning of tabs according to tree structure.
+    """
+
+    def __init__(self, win_id, parent=None):
+        # root of the tab tree, common for all tabs in the window
+        self.tree_root = Node(None)
+        super().__init__(win_id, parent)
+        self.tabBar().tabMoved.disconnect(self.update_tab_titles)
+
+    def _init_config(self):
+        super()._init_config()
+        # For tree-tabs
+        self.update_tab_titles()  # Must also be called when deactivating
+        self.tree_tab_update()
+
+    def get_tab_fields(self, idx):
+        """Add tree field data to normal tab field data."""
+        fields = super().get_tab_fields(idx)
+
+        tab = self.widget(idx)
+        fields['collapsed'] = '[...] ' if tab.node.collapsed else ''
+
+        # we remove the first two chars because every tab is child of tree
+        # root and that gets rendered as well
+        rendered_tree = self.tree_root.render()
+        try:
+            pre, _ = rendered_tree[idx+1]
+            tree_prefix = pre[2:]
+        except IndexError:  # window or first tab are not initialized yet
+            tree_prefix = ""
+            log.misc.error("tree_prefix is empty!")
+
+        fields['tree'] = tree_prefix
+        return fields
+
+    def update_tree_tab_positions(self):
+        """Update tab positions according to the tree structure."""
+        nodes = self.tree_root.traverse(render_collapsed=False)
+        for idx, node in enumerate(nodes):
+            if idx > 0:
+                cur_idx = self.indexOf(node.value)
+                self.tabBar().moveTab(cur_idx, idx-1)
+
+    def update_tree_tab_visibility(self):
+        """Hide collapsed tabs and show uncollapsed ones.
+
+        Sync the internal tree to the tabs the user can actually see.
+        """
+        for node in self.tree_root.traverse():
+            if node.value is None:
+                continue
+            if any(ancestor.collapsed for ancestor in node.path[:-1]):
+                if self.indexOf(node.value) != -1:
+                    # node should be hidden but is shown
+                    cur_tab = node.value
+                    idx = self.indexOf(cur_tab)
+                    if idx != -1:
+                        self.removeTab(idx)
+            else:
+                if self.indexOf(node.value) == -1:
+                    # node should be shown but is hidden
+                    parent = node.parent
+                    tab = node.value
+                    name = tab.title()
+                    icon = tab.icon()
+                    if node.parent is not None:
+                        parent_idx = self.indexOf(node.parent.value)
+                    self.insertTab(parent_idx + 1, tab, icon, name)
+                    tab.node.parent = parent  # insertTab resets node
+
+    def tree_tab_update(self):
+        """Update titles and positions."""
+        self.update_tree_tab_visibility()
+        self.update_tree_tab_positions()
+        self.update_tab_titles()
diff --git a/qutebrowser/misc/notree.py b/qutebrowser/misc/notree.py
new file mode 100644
index 00000000000..9d180ad4ce7
--- /dev/null
+++ b/qutebrowser/misc/notree.py
@@ -0,0 +1,354 @@
+# vim: ft=python fileencoding=utf-8 sts=4 sw=4 et:
+
+# Copyright 2019 Giuseppe Stelluto (pinusc) <giuseppe@gstelluto.com>
+#
+# This file is part of qutebrowser.
+#
+# qutebrowser is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# qutebrowser is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with qutebrowser.  If not, see <https://www.gnu.org/licenses/>.
+
+"""Tree library for tree-tabs.
+
+The fundamental unit is the Node class.
+
+Create a tree with with Node(value, parent):
+root = Node('foo')
+child = Node('bar', root)
+child2 = Node('baz', root)
+child3 = Node('lorem', child)
+
+You can also assign parent after instantiation, or even reassign it:
+child4 = Node('ipsum')
+child4.parent = root
+
+Assign children:
+child.children = []
+child2.children = [child4, child3]
+child3.parent
+> Node('foo/bar/baz')
+
+Render a tree with render_tree(root_node):
+render_tree(root)
+
+> ('', 'foo')
+> ('├─', 'bar')
+> ('│ ├─', 'lorem')
+> ('│ └─', 'ipsum')
+> ('└─', 'baz')
+"""
+import enum
+from typing import Optional, TypeVar, Sequence, List, Tuple, Iterable, Generic
+import itertools
+
+# For Node.render
+CORNER = '└─'
+INTERSECTION = '├─'
+PIPE = '│'
+
+
+class TreeError(RuntimeError):
+    """Exception used for tree-related errors."""
+
+
+class TraverseOrder(enum.Enum):
+    """To be used as argument to traverse().
+
+    Implemented orders are pre-order and post-order.
+    Attributes:
+        PRE: pre-order (parents before children). Same as in Node.render
+        POST: children of a node are always yield before their parent.
+        POST_R: Like POST, but children are yield in reverse order
+    """
+
+    PRE = 'pre-order'
+    POST = 'post-order'
+    POST_R = 'post-order-reverse'
+
+
+uid_gen = itertools.count(0)
+
+# generic type of value held by Node
+T = TypeVar('T')  # pylint: disable=invalid-name
+
+
+class Node(Generic[T]):
+    """Fundamental unit of notree library.
+
+    Attributes:
+        value: The element (ususally a tab) the node represents
+        parent: Node's parent.
+        children: Node's children elements.
+        siblings: Children of parent node that are not self.
+        path: List of nodes from root of tree to self value, parent, and
+            children can all be set by user. Everything else will be updated
+            accordingly, so that if `node.parent = root_node`, then `node in
+            root_node.children` will be True.
+    """
+
+    sep: str = '/'
+    __parent: Optional['Node[T]'] = None
+    # this is a global 'static' class attribute
+
+    def __init__(self,
+                 value: T,
+                 parent: Optional['Node[T]'] = None,
+                 childs: Sequence['Node[T]'] = (),
+                 uid: Optional[int] = None) -> None:
+        if uid is not None:
+            self.__uid = uid
+        else:
+            self.__uid = next(uid_gen)
+
+        self.value = value
+        # set initial values so there's no need for AttributeError checks
+        self.__parent: Optional['Node[T]'] = None
+        self.__children: List['Node[T]'] = []
+
+        # For render memoization
+        self.__modified = False
+        self.__set_modified()  # not the same as line above
+        self.__rendered: Optional[List[Tuple[str, 'Node[T]']]] = None
+
+        if parent:
+            self.parent = parent  # calls setter
+        if childs:
+            self.children = childs  # this too
+
+        self.__collapsed = False
+
+    @property
+    def uid(self) -> int:
+        return self.__uid
+
+    @property
+    def parent(self) -> Optional['Node[T]']:
+        return self.__parent
+
+    @parent.setter
+    def parent(self, value: 'Node[T]') -> None:
+        """Set parent property. Also adds self to value.children."""
+        # pylint: disable=protected-access
+        assert(value is None or isinstance(value, Node))
+        if self.__parent:
+            self.__parent.__disown(self)
+            self.__parent = None
+        if value is not None:
+            value.__add_child(self)
+            self.__parent = value
+        self.__set_modified()
+
+    @property
+    def children(self) -> Sequence['Node[T]']:
+        return tuple(self.__children)
+
+    @children.setter
+    def children(self, value: Sequence['Node[T]']) -> None:
+        """Set children property, preserving order.
+
+        Also sets n.parent = self for n in value. Does not allow duplicates.
+        """
+        seen = set(value)
+        if len(seen) != len(value):
+            raise TreeError("A duplicate item is present in in %r" % value)
+        new_children = list(value)
+        for child in new_children:
+            if child.parent is not self:
+                child.parent = self
+        self.__children = new_children
+        self.__set_modified()
+
+    @property
+    def path(self) -> List['Node[T]']:
+        """Get a list of all nodes from the root node to self."""
+        if self.parent is None:
+            return [self]
+        else:
+            return self.parent.path + [self]
+
+    @property
+    def depth(self) -> int:
+        """Get the number of nodes between self and the root node."""
+        return len(self.path) - 1
+
+    @property
+    def index(self) -> int:
+        """Get self's position among its siblings (self.parent.children)."""
+        if self.parent is not None:
+            return self.parent.children.index(self)
+        else:
+            raise TreeError('Node has no parent.')
+
+    @property
+    def collapsed(self) -> bool:
+        return self.__collapsed
+
+    @collapsed.setter
+    def collapsed(self, val: bool) -> None:
+        self.__collapsed = val
+        self.__set_modified()
+
+    def __set_modified(self) -> None:
+        """If self is modified, every ancestor is modified as well."""
+        for node in self.path:
+            node.__modified = True  # pylint: disable=protected-access
+
+    def render(self) -> List[Tuple[str, 'Node[T]']]:
+        """Render a tree with ascii symbols.
+
+        Tabs appear in the same order as in traverse() with TraverseOrder.PRE
+        Args:
+            node; the root of the tree to render
+
+        Return: list of tuples where the first item is the symbol,
+                and the second is the node it refers to
+        """
+        if not self.__modified and self.__rendered is not None:
+            return self.__rendered
+
+        result = [('', self)]
+        for child in self.children:
+            if child.children:
+                subtree = child.render()
+                if child is not self.children[-1]:
+                    subtree = [(PIPE + ' ' + c, n) for c, n in subtree]
+                    char = INTERSECTION
+                else:
+                    subtree = [('  ' + c, n) for c, n in subtree]
+                    char = CORNER
+                subtree[0] = (char, subtree[0][1])
+                if child.collapsed:
+                    result += [subtree[0]]
+                else:
+                    result += subtree
+            else:
+                if child is self.children[-1]:
+                    result.append((CORNER, child))
+                else:
+                    result.append((INTERSECTION, child))
+        self.__modified = False
+        self.__rendered = list(result)
+        return list(result)
+
+    def traverse(self, order: TraverseOrder = TraverseOrder.PRE,
+                 render_collapsed: bool = True) -> Iterable['Node']:
+        """Generator for all descendants of `self`.
+
+        Args:
+            order: a TraverseOrder object. See TraverseOrder documentation.
+            render_collapsed: whether to yield children of collapsed nodes
+            Even if render_collapsed is False, collapsed nodes are be rendered.
+            It's their children that won't.
+        """
+        if order == TraverseOrder.PRE:
+            yield self
+
+        if self.collapsed and not render_collapsed:
+            if order != TraverseOrder.PRE:
+                yield self
+            return
+
+        f = reversed if order is TraverseOrder.POST_R else lambda x: x
+        for child in f(self.children):
+            if render_collapsed or not child.collapsed:
+                yield from child.traverse(order, render_collapsed)
+            else:
+                yield child
+        if order in [TraverseOrder.POST, TraverseOrder.POST_R]:
+            yield self
+
+    def __add_child(self, node: 'Node[T]') -> None:
+        if node not in self.__children:
+            self.__children.append(node)
+
+    def __disown(self, value: 'Node[T]') -> None:
+        self.__set_modified()
+        if value in self.__children:
+            self.__children.remove(value)
+
+    def get_descendent_by_uid(self, uid: int) -> Optional['Node[T]']:
+        """Return descendent identified by the provided uid.
+
+        Returns None if there is no such descendent.
+
+        Args:
+            uid: The uid of the node to return
+        """
+        for descendent in self.traverse():
+            if descendent.uid == uid:
+                return descendent
+        return None
+
+    def promote(self, times: int = 1, to: str = 'first') -> None:
+        """Makes self a child of its grandparent, i.e. sibling of its parent.
+
+        Args:
+            times: How many levels to promote the tab to. to: One of 'next',
+            'prev', 'first', 'last'. Determines the position among siblings
+            after being promoted. 'next' and 'prev' are relative to the current
+            parent.
+
+        """
+        if to not in ['first', 'last', 'next', 'prev']:
+            raise Exception("Invalid value supplied for 'to': " + to)
+        position = {'first': 0, 'last': -1}.get(to, None)
+        diff = {'next': 1, 'prev': 0}.get(to, 1)
+        count = times
+        while count > 0:
+            if self.parent is None or self.parent.parent is None:
+                raise TreeError("Tab has no parent!")
+            grandparent = self.parent.parent
+            if position is not None:
+                idx = position
+            else:  # diff is necessarily not none
+                idx = self.parent.index + diff
+            self.parent = None
+
+            siblings = list(grandparent.children)
+            if idx != -1:
+                siblings.insert(idx, self)
+            else:
+                siblings.append(self)
+            grandparent.children = siblings
+            count -= 1
+
+    def demote(self, to: str = 'last') -> None:
+        """Demote a tab making it a child of its previous adjacent sibling."""
+        if self.parent is None or self.parent.children is None:
+            raise TreeError("Tab has no siblings!")
+        siblings = list(self.parent.children)
+
+        # we want previous node in the same subtree as current node
+        rel_idx = siblings.index(self) - 1
+
+        if rel_idx >= 0:
+            parent = siblings[rel_idx]
+            new_siblings = list(parent.children)
+            position = {'first': 0, 'last': -1}.get(to, -1)
+            if position == 0:
+                new_siblings.insert(0, self)
+            else:
+                new_siblings.append(self)
+            parent.children = new_siblings
+        else:
+            raise TreeError("Tab has no previous sibling!")
+
+    def __repr__(self) -> str:
+        try:
+            value = str(self.value.url().url())  # type: ignore
+        except Exception:
+            value = str(self.value)
+        return "<Node -%d- '%s'>" % (self.__uid, value)
+
+    def __str__(self) -> str:
+        # return "<Node '%s'>" % self.value
+        return str(self.value)
diff --git a/qutebrowser/misc/sessions.py b/qutebrowser/misc/sessions.py
index 3608568766c..c8445e4ad4d 100644
--- a/qutebrowser/misc/sessions.py
+++ b/qutebrowser/misc/sessions.py
@@ -243,6 +243,9 @@ def _save_tab(self, tab, active, with_history=True):
             active: Whether the tab is currently active.
             with_history: Include the tab's history.
         """
+        # FIXME understand why this happens
+        if tab is None:
+            return {}
         data: _JsonType = {'history': []}
         if active:
             data['active'] = True
@@ -287,13 +290,29 @@ def _save_all(self, *, only_window=None, with_private=False, with_history=True):
             if getattr(active_window, 'win_id', None) == win_id:
                 win_data['active'] = True
             win_data['geometry'] = bytes(main_window.saveGeometry())
-            win_data['tabs'] = []
             if tabbed_browser.is_private:
                 win_data['private'] = True
-            for i, tab in enumerate(tabbed_browser.widgets()):
-                active = i == tabbed_browser.widget.currentIndex()
-                win_data['tabs'].append(self._save_tab(tab, active,
-                                                       with_history=with_history))
+
+            if tabbed_browser.is_treetabbedbrowser:
+                # a dict where keys are node UIDs, and values are dicts
+                # with tab data (the result of _save_tab) and a list of
+                # children UIDs
+                tree_data = {}
+                root_node = tabbed_browser.widget.tree_root
+                for i, node in enumerate(root_node.traverse(), -1):
+                    node_data = {}
+                    active = i == tabbed_browser.widget.currentIndex()
+                    node_data['tab'] = self._save_tab(node.value, active)
+                    node_data['children'] = [c.uid for c in node.children]
+                    node_data['collapsed'] = node.collapsed
+                    tree_data[node.uid] = node_data
+                win_data['tree'] = tree_data
+            else:
+                win_data['tabs'] = []
+                for i, tab in enumerate(tabbed_browser.widgets()):
+                    active = i == tabbed_browser.widget.currentIndex()
+                    win_data['tabs'].append(self._save_tab(tab, active,
+                                                           with_history=with_history))
             data['windows'].append(win_data)
         return data

@@ -461,6 +480,45 @@ def _load_tab(self, new_tab, data):  # noqa: C901
         except ValueError as e:
             raise SessionError(e)

+    def _load_tree(self, tabbed_browser, tree_data):
+        tree_keys = list(tree_data.keys())
+        if not tree_keys:
+            return None
+
+        root_data = tree_data.get(tree_keys[0])
+        if root_data is None:
+            return None
+
+        root_node = tabbed_browser.widget.tree_root
+        tab_to_focus = None
+        index = -1
+
+        def recursive_load_node(uid):
+            nonlocal tab_to_focus
+            nonlocal index
+            index += 1
+            node_data = tree_data[uid]
+            children_uids = node_data['children']
+
+            if tree_data[uid]['tab'].get('active'):
+                tab_to_focus = index
+
+            tab_data = node_data['tab']
+            new_tab = tabbed_browser.tabopen(background=False)
+            self._load_tab(new_tab, tab_data)
+
+            new_tab.node.parent = root_node
+            children = [recursive_load_node(uid) for uid in children_uids]
+            new_tab.node.children = children
+            new_tab.node.collapsed = node_data['collapsed']
+            return new_tab.node
+
+        for child_uid in root_data['children']:
+            child = recursive_load_node(child_uid)
+            child.parent = root_node
+
+        return tab_to_focus
+
     def _load_window(self, win):
         """Turn yaml data into windows."""
         window = mainwindow.MainWindow(geometry=win['geometry'],
@@ -468,13 +526,29 @@ def _load_window(self, win):
         tabbed_browser = objreg.get('tabbed-browser', scope='window',
                                     window=window.win_id)
         tab_to_focus = None
-        for i, tab in enumerate(win['tabs']):
-            new_tab = tabbed_browser.tabopen(background=False)
-            self._load_tab(new_tab, tab)
-            if tab.get('active', False):
-                tab_to_focus = i
-            if new_tab.data.pinned:
-                new_tab.set_pinned(True)
+
+        # plain_tabs is used in case the saved session contains a tree and
+        # tree-tabs is not enabled, or if the saved session contains normal
+        # tabs
+        plain_tabs = win.get('tabs', None)
+        if win.get('tree'):
+            if tabbed_browser.is_treetabbedbrowser:
+                tree_data = win.get('tree')
+                tab_to_focus = self._load_tree(tabbed_browser, tree_data)
+                tabbed_browser.widget.tree_tab_update()
+            else:
+                tree = win.get('tree')
+                plain_tabs = [tree[i]['tab'] for i in tree if
+                              tree[i]['tab']]
+        if plain_tabs:
+            for i, tab in enumerate(plain_tabs):
+                new_tab = tabbed_browser.tabopen(background=False)
+                self._load_tab(new_tab, tab)
+                if tab.get('active', False):
+                    tab_to_focus = i
+                if new_tab.data.pinned:
+                    new_tab.set_pinned(True)
+
         if tab_to_focus is not None:
             tabbed_browser.widget.setCurrentIndex(tab_to_focus)

diff --git a/tests/end2end/features/conftest.py b/tests/end2end/features/conftest.py
index 8ce8ba699a1..75bddfa0f8f 100644
--- a/tests/end2end/features/conftest.py
+++ b/tests/end2end/features/conftest.py
@@ -214,12 +214,15 @@ def open_path(quteproc, server, path):
     path = path.replace('(port)', str(server.port))

     new_tab = False
+    related_tab = False
     new_bg_tab = False
     new_window = False
     private = False
     as_url = False
     wait = True

+    related_tab_suffix = ' in a new related tab'
+    related_background_tab_suffix = ' in a new related background tab'
     new_tab_suffix = ' in a new tab'
     new_bg_tab_suffix = ' in a new background tab'
     new_window_suffix = ' in a new window'
@@ -231,6 +234,14 @@ def open_path(quteproc, server, path):
         if path.endswith(new_tab_suffix):
             path = path[:-len(new_tab_suffix)]
             new_tab = True
+        elif path.endswith(related_tab_suffix):
+            path = path[:-len(related_tab_suffix)]
+            new_tab = True
+            related_tab = True
+        elif path.endswith(related_background_tab_suffix):
+            path = path[:-len(related_background_tab_suffix)]
+            new_bg_tab = True
+            related_tab = True
         elif path.endswith(new_bg_tab_suffix):
             path = path[:-len(new_bg_tab_suffix)]
             new_bg_tab = True
@@ -249,9 +260,9 @@ def open_path(quteproc, server, path):
         else:
             break

-    quteproc.open_path(path, new_tab=new_tab, new_bg_tab=new_bg_tab,
-                       new_window=new_window, private=private, as_url=as_url,
-                       wait=wait)
+    quteproc.open_path(path, related_tab=related_tab, new_tab=new_tab,
+                       new_bg_tab=new_bg_tab, new_window=new_window,
+                       private=private, as_url=as_url, wait=wait)


 @bdd.when(bdd.parsers.parse("I set {opt} to {value}"))
diff --git a/tests/end2end/fixtures/quteprocess.py b/tests/end2end/fixtures/quteprocess.py
index c3194b8397e..a59a05ec222 100644
--- a/tests/end2end/fixtures/quteprocess.py
+++ b/tests/end2end/fixtures/quteprocess.py
@@ -651,16 +651,17 @@ def temp_setting(self, opt, value):
         self.set_setting(opt, old_value)

     def open_path(self, path, *, new_tab=False, new_bg_tab=False,
-                  new_window=False, private=False, as_url=False, port=None,
-                  https=False, wait=True):
+                  related_tab=False, new_window=False, private=False,
+                  as_url=False, port=None, https=False, wait=True):
         """Open the given path on the local webserver in qutebrowser."""
         url = self.path_to_url(path, port=port, https=https)
         self.open_url(url, new_tab=new_tab, new_bg_tab=new_bg_tab,
-                      new_window=new_window, private=private, as_url=as_url,
-                      wait=wait)
+                      related_tab=related_tab, new_window=new_window,
+                      private=private, as_url=as_url, wait=wait)

     def open_url(self, url, *, new_tab=False, new_bg_tab=False,
-                 new_window=False, private=False, as_url=False, wait=True):
+                 related_tab=False, new_window=False, private=False,
+                 as_url=False, wait=True):
         """Open the given url in qutebrowser."""
         if sum(1 for opt in [new_tab, new_bg_tab, new_window, private, as_url]
                if opt) > 1:
@@ -670,9 +671,15 @@ def open_url(self, url, *, new_tab=False, new_bg_tab=False,
             self.send_cmd(url, invalid=True)
             line = None
         elif new_tab:
-            line = self.send_cmd(':open -t ' + url)
+            if related_tab:
+                line = self.send_cmd(':open -t -r ' + url)
+            else:
+                line = self.send_cmd(':open -t ' + url)
         elif new_bg_tab:
-            line = self.send_cmd(':open -b ' + url)
+            if related_tab:
+                line = self.send_cmd(':open -b -r ' + url)
+            else:
+                line = self.send_cmd(':open -b ' + url)
         elif new_window:
             line = self.send_cmd(':open -w ' + url)
         elif private:
diff --git a/tests/helpers/stubs.py b/tests/helpers/stubs.py
index 341d1de2a61..2dfb15916e9 100644
--- a/tests/helpers/stubs.py
+++ b/tests/helpers/stubs.py
@@ -337,6 +337,7 @@ class FakeCommand:
     hide: bool = False
     debug: bool = False
     deprecated: bool = False
+    tree_tab: bool = False
     completion: Any = None
     maxsplit: int = None
     takes_count: Callable[[], bool] = lambda: False
diff --git a/tests/unit/misc/test_notree.py b/tests/unit/misc/test_notree.py
new file mode 100644
index 00000000000..95f92f1a68f
--- /dev/null
+++ b/tests/unit/misc/test_notree.py
@@ -0,0 +1,308 @@
+# vim: ft=python fileencoding=utf-8 sts=4 sw=4 et:
+
+# Copyright 2019 Florian Bruhin (The-Compiler) <me@the-compiler.org>
+#
+# This file is part of qutebrowser.
+#
+# qutebrowser is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# qutebrowser is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with qutebrowser.  If not, see <https://www.gnu.org/licenses/>.
+"""Tests for misc.notree library."""
+import pytest
+
+from qutebrowser.misc.notree import TreeError, Node, TraverseOrder
+
+
+@pytest.fixture
+def tree():
+    """Return an example tree.
+
+    n1
+    ├─n2
+    │ ├─n4
+    │ └─n5
+    └─n3
+      ├─n6
+      │ ├─n7
+      │ ├─n8
+      │ └─n9
+      │   └─n10
+      └─n11
+    """
+    # these are actually used because they appear in expected strings
+    n1 = Node('n1')
+    n2 = Node('n2', n1)
+    n4 = Node('n4', n2)
+    n5 = Node('n5', n2)
+    n3 = Node('n3', n1)
+    n6 = Node('n6', n3)
+    n7 = Node('n7', n6)
+    n8 = Node('n8', n6)
+    n9 = Node('n9', n6)
+    n10 = Node('n10', n9)
+    n11 = Node('n11', n3)
+    return n1, n2, n3, n4, n5, n6, n7, n8, n9, n10, n11
+
+
+@pytest.fixture
+def node(tree):
+    return tree[0]
+
+
+def test_creation():
+    node = Node('foo')
+    assert node.value == 'foo'
+
+    child = Node('bar', node)
+    assert child.parent == node
+    assert node.children == (child, )
+
+
+def test_attach_parent():
+    n1 = Node('n1', None, [])
+    print(n1.children)
+    n2 = Node('n2', n1)
+    n3 = Node('n3')
+
+    n2.parent = n3
+    assert n2.parent == n3
+    assert n3.children == (n2, )
+    assert n1.children == ()
+
+
+def test_duplicate_child():
+    p = Node('n1')
+    try:
+        c1 = Node('c1', p)
+        c2 = Node('c2', p)
+        p.children = [c1, c1, c2]
+        raise AssertionError("Can add duplicate child")
+    except TreeError:
+        pass
+    finally:
+        if len(p.children) == 3:
+            raise AssertionError("Can add duplicate child")
+
+
+def test_replace_parent():
+    p1 = Node('foo')
+    p2 = Node('bar')
+    _ = Node('_', p2)
+    c = Node('baz', p1)
+    c.parent = p2
+    assert c.parent is p2
+    assert c not in p1.children
+    assert c in p2.children
+
+
+def test_replace_children(tree):
+    n2 = tree[1]
+    n3 = tree[2]
+    n6 = tree[5]
+    n11 = tree[10]
+    n3.children = [n11]
+    n2.children = (n6, ) + n2.children
+    assert n6.parent is n2
+    assert n6 in n2.children
+    assert n11.parent is n3
+    assert n11 in n3.children
+    assert n6 not in n3.children
+    assert len(n3.children) == 1
+
+
+def test_promote_to_first(tree):
+    n1 = tree[0]
+    n3 = tree[2]
+    n6 = tree[5]
+    assert n6.parent is n3
+    assert n3.parent is n1
+    n6.promote(to='first')
+    assert n6.parent is n1
+    assert n1.children[0] is n6
+
+
+def test_promote_to_last(tree):
+    n1 = tree[0]
+    n3 = tree[2]
+    n6 = tree[5]
+    assert n6.parent is n3
+    assert n3.parent is n1
+    n6.promote(to='last')
+    assert n6.parent is n1
+    assert n1.children[-1] is n6
+
+
+def test_promote_to_prev(tree):
+    n1 = tree[0]
+    n3 = tree[2]
+    n6 = tree[5]
+    assert n6.parent is n3
+    assert n3.parent is n1
+    assert n1.children[1] is n3
+    n6.promote(to='prev')
+    assert n6.parent is n1
+    assert n1.children[1] is n6
+
+
+def test_promote_to_next(tree):
+    n1 = tree[0]
+    n3 = tree[2]
+    n6 = tree[5]
+    assert n6.parent is n3
+    assert n3.parent is n1
+    assert n1.children[1] is n3
+    n6.promote(to='next')
+    assert n6.parent is n1
+    assert n1.children[2] is n6
+
+
+def test_demote_to_first(tree):
+    n11 = tree[10]
+    n6 = tree[5]
+    assert n11.parent is n6.parent
+    parent = n11.parent
+    assert parent.children.index(n11) == parent.children.index(n6) + 1
+    n11.demote(to='first')
+    assert n11.parent is n6
+    assert n6.children[0] is n11
+
+
+def test_demote_to_last(tree):
+    n11 = tree[10]
+    n6 = tree[5]
+    assert n11.parent is n6.parent
+    parent = n11.parent
+    assert parent.children.index(n11) == parent.children.index(n6) + 1
+    n11.demote(to='last')
+    assert n11.parent is n6
+    assert n6.children[-1] is n11
+
+
+def test_traverse(node):
+    len_traverse = len(list(node.traverse()))
+    len_render = len(node.render())
+    assert len_traverse == len_render
+
+
+def test_traverse_postorder(tree):
+    n1, n2, n3, n4, n5, n6, n7, n8, n9, n10, n11 = tree
+    actual = list(n1.traverse(TraverseOrder.POST))
+    print('\n'.join([str(n) for n in actual]))
+    assert actual == [n4, n5, n2, n7, n8, n10, n9, n6, n11, n3, n1]
+
+
+def test_traverse_postorder_r(tree):
+    n1, n2, n3, n4, n5, n6, n7, n8, n9, n10, n11 = tree
+    actual = list(n1.traverse(TraverseOrder.POST_R))
+    print('\n'.join([str(n) for n in actual]))
+    assert actual == [n11, n10, n9, n8, n7, n6, n3, n5, n4, n2, n1]
+
+
+def test_render_tree(node):
+    expected = [
+        'n1',
+        '├─n2',
+        '│ ├─n4',
+        '│ └─n5',
+        '└─n3',
+        '  ├─n6',
+        '  │ ├─n7',
+        '  │ ├─n8',
+        '  │ └─n9',
+        '  │   └─n10',
+        '  └─n11'
+    ]
+    result = [char + str(n) for char, n in node.render()]
+    print('\n'.join(result))
+    assert expected == result
+
+
+def test_uid(node):
+    uids = set()
+    for n in node.traverse():
+        assert n not in uids
+        uids.add(n.uid)
+    # pylint: disable=unused-variable
+    n1 = Node('n1')
+    n2 = Node('n2', n1)
+    n4 = Node('n4', n2)  # noqa: F841
+    n5 = Node('n5', n2)  # noqa: F841
+    n3 = Node('n3', n1)
+    n6 = Node('n6', n3)
+    n7 = Node('n7', n6)  # noqa: F841
+    n8 = Node('n8', n6)  # noqa: F841
+    n9 = Node('n9', n6)
+    n10 = Node('n10', n9)  # noqa: F841
+    n11 = Node('n11', n3)
+    # pylint: enable=unused-variable
+    for n in n1.traverse():
+        assert n not in uids
+        uids.add(n.uid)
+
+    n11_uid = n11.uid
+    assert n1.get_descendent_by_uid(n11_uid) is n11
+    assert node.get_descendent_by_uid(n11_uid) is None
+
+
+def test_collapsed(node):
+    pre_collapsed_traverse = list(node.traverse())
+    to_collapse = node.children[1]
+
+    # collapse
+    to_collapse.collapsed = True
+    assert to_collapse.collapsed is True
+    for n in node.traverse(render_collapsed=False):
+        assert to_collapse not in n.path[:-1]
+
+    assert list(to_collapse.traverse(render_collapsed=False)) == [to_collapse]
+
+    assert list(node.traverse()) == pre_collapsed_traverse
+
+    expected = [
+        'n1',
+        '├─n2',
+        '│ ├─n4',
+        '│ └─n5',
+        '└─n3'
+    ]
+    result = [char + str(n) for char, n in node.render()]
+    print('\n'.join(result))
+    assert expected == result
+
+    # uncollapse
+    to_collapse.collapsed = False
+
+    assert any(n for n in node.traverse(render_collapsed=False) if to_collapse
+               in n.path[:-1])
+
+
+def test_memoization(node):
+    assert node._Node__modified is True
+    node.render()
+    assert node._Node__modified is False
+
+    node.children[0].parent = None
+    assert node._Node__modified is True
+    node.render()
+    assert node._Node__modified is False
+
+    n2 = Node('ntest', parent=node)
+    assert node._Node__modified is True
+    assert n2._Node__modified is True
+    node.render()
+    assert node._Node__modified is False
+
+    node.children[0].children[1].parent = None
+    assert node._Node__modified is True
+    assert node.children[0]._Node__modified is True
+    node.render()
+    assert node._Node__modified is False