Make `select_on_remove=prev` select previous sibling

Updates `tabs.select_on_remove=prev` for tree tab windows so that the
previous sibling is selected, instead of the previous tab in the tab
bar. I've found that the selected tab descending to the rightmost leaf
node of a tab group when I close a top level tab is never what I want.

It may have been more respectful of people's preferences and habits to
add a "previous-sibling" option for the `tabs.select_on_remove` option,
but that would be a fair bit of work. Turning that option into strings
and mapping them to QTabBar enum values at the only place they are used
wouldn't be so bad, but then there would be another place where non-tree
code would have to deal with a tree specific option. So I figured I
would be bold and change the default behaviour and see if anyone
complained.

qutebrowser/mainwindow/treetabbedbrowser.py

@@ -15,6 +15,7 @@ from qutebrowser.mainwindow.tabbedbrowser import TabbedBrowser, _UndoEntry
 from qutebrowser.mainwindow.treetabwidget import TreeTabWidget
 from qutebrowser.browser import browsertab
 from qutebrowser.misc import notree
+from qutebrowser.qt.widgets import QTabBar
 
 
 @dataclasses.dataclass
@@ -152,6 +153,31 @@ class TreeTabbedBrowser(TabbedBrowser):
 
         node = tab.node
         parent = node.parent
+        current_tab = self.current_tab()
+
+        # Override tabs.select_on_remove behavior to be tree aware.
+        # The default behavior is in QTabBar.removeTab(), by way of
+        # QTabWidget.removeTab(). But here we are detaching the tab from the
+        # tree before those methods get called, so if we want to have a tree
+        # aware behavior we need to implement that here by selecting the new
+        # tab before the closing the current one.
+        if tab == current_tab:
+            selection_behavior = self.widget.tabBar().selectionBehaviorOnRemove()
+            # Given a tree structure like:
+            # - one
+            #   - two
+            # - three (active)
+            # If the setting is "prev" (aka left) we want to end up with tab
+            # "one" selected after closing tab "three". Switch to either the
+            # current tab's previous sibling or its parent.
+            if selection_behavior == QTabBar.SelectionBehavior.SelectLeftTab:
+                siblings = parent.children
+                rel_index = siblings.index(node)
+                if rel_index == 0:
+                    next_tab = parent.value
+                else:
+                    next_tab = siblings[rel_index-1].value
+                self.widget.setCurrentWidget(next_tab)
 
         if node.collapsed:
             # Collapsed nodes have already been removed from the TabWidget so

tests/end2end/features/treetabs.feature

@@ -356,3 +356,43 @@ Feature: Tree tab management
           - about:blank?grandparent
             - about:blank?parent (active)
           """
+
+    Scenario: Tabs.select_on_remove prev selects previous sibling
+        When I open about:blank?one
+        And I open about:blank?two in a new related tab
+        And I open about:blank?three in a new tab
+        And I run :set tabs.select_on_remove prev
+        And I run :tab-close
+        And I run :config-unset tabs.select_on_remove
+        Then the following tabs should be open:
+          """
+          - about:blank?one (active)
+            - about:blank?two
+          """
+
+    Scenario: Tabs.select_on_remove prev selects parent
+        When I open about:blank?one
+        And I open about:blank?two in a new related tab
+        And I open about:blank?three in a new sibling tab
+        And I run :set tabs.select_on_remove prev
+        And I run :tab-close
+        And I run :config-unset tabs.select_on_remove
+        Then the following tabs should be open:
+          """
+          - about:blank?one (active)
+            - about:blank?two
+          """
+
+    Scenario: Tabs.select_on_remove prev can be overridden
+        When I open about:blank?one
+        And I open about:blank?two in a new related tab
+        And I open about:blank?three in a new tab
+        And I run :tab-select ?two
+        And I run :set tabs.select_on_remove prev
+        And I run :tab-close --next
+        And I run :config-unset tabs.select_on_remove
+        Then the following tabs should be open:
+          """
+          - about:blank?one
+          - about:blank?three (active)
+          """