Scroll tab bar so current tab isn't at an edge

It is unfortunate that everything we could override to fix the "gravity"
of the visible portion of the tab bar is in QTabBarPrivate. Also that it
has no public API for scrolling ... except for the buttons.

Gets the QToolButtons that are children of the QTabWidget (actually of
the QTabBarPrivate) and uses them to scroll the tab bar. Only works
because they aren't hidden, they are just of zero size.

Gets the tab indices at the edges of the tab bar and compares them to
the current tab and if there is less than 5 tabs differnce it scrolls
across to make up the difference. I didn't bother making the padding
count configurable because I doubt this'll get merged.

The throttle is mostly for resizing and shutting down (not sure why that
is not a decorator).

qutebrowser/mainwindow/tabwidget.py

@@ -13,13 +13,14 @@ from qutebrowser.qt.core import (pyqtSignal, pyqtSlot, Qt, QSize, QRect, QPoint,
                           QTimer, QUrl)
 from qutebrowser.qt.widgets import (QTabWidget, QTabBar, QSizePolicy, QProxyStyle,
                              QStyle, QStylePainter, QStyleOptionTab,
-                             QCommonStyle)
+                             QCommonStyle, QToolButton)
 from qutebrowser.qt.gui import QIcon, QPalette, QColor
 
 from qutebrowser.utils import qtutils, objreg, utils, usertypes, log
 from qutebrowser.config import config, stylesheet
-from qutebrowser.misc import objects, debugcachestats
+from qutebrowser.misc import objects, debugcachestats, throttle
 from qutebrowser.browser import browsertab
+from qutebrowser.qt import sip
 
 
 class TabWidget(QTabWidget):
@@ -42,6 +43,7 @@ class TabWidget(QTabWidget):
 
     def __init__(self, win_id, parent=None):
         super().__init__(parent)
+        self.scroll_tab_bar = throttle.Throttle(self._scroll_tab_bar, 100)
         bar = TabBar(win_id, self)
         self.setStyle(TabBarStyle())
         self.setTabBar(bar)
@@ -56,6 +58,7 @@ class TabWidget(QTabWidget):
         bar.setDrawBase(False)
         self._init_config()
         config.instance.changed.connect(self._init_config)
+        self._scroll_left, self._scroll_right = self.findChildren(QToolButton)
 
     @config.change_filter('tabs')
     def _init_config(self):
@@ -316,6 +319,49 @@ class TabWidget(QTabWidget):
         self.tab_bar().on_current_changed()
         self.update_tab_titles()
         self.tab_index_changed.emit(index, self.count())
+        QTimer.singleShot(0, self.scroll_tab_bar)
+
+    def _scroll_tab_bar(self):
+        """Scroll tab bar so that the current tab is not at an edge."""
+        if sip.isdeleted(self):
+            # probably called async while shutting down
+            return
+
+        num_tabs = 5  # padding required
+        idx = self.currentIndex()
+        if idx < 0:
+            return
+
+        position = config.cache['tabs.position']
+        if position == QTabWidget.TabPosition.North:
+            start = self.rect().topLeft()
+            end = self.rect().topRight()
+        elif position == QTabWidget.TabPosition.South:
+            start = self.rect().bottomLeft()
+            end = self.rect().bottomRight()
+        elif position == QTabWidget.TabPosition.West:
+            start = self.rect().topLeft()
+            end = self.rect().bottomLeft()
+        elif position == QTabWidget.TabPosition.East:
+            start = self.rect().topRight()
+            end = self.rect().bottomRight()
+
+        start_idx = self.tabBar().tabAt(start)
+        end_idx = self.tabBar().tabAt(end)
+        if start_idx < 0 or end_idx < 0:
+            log.misc.warning("Could not get tabs at edges of tab bar.")
+            return
+
+        count = num_tabs - (end_idx - idx)
+        if count > 0:
+            for _ in range(count):
+                self._scroll_right.click()
+            return
+        count = idx - start_idx
+        if count < num_tabs:
+            for _ in range(num_tabs - count):
+                self._scroll_left.click()
+            return
 
     @pyqtSlot()
     def _on_new_tab_requested(self):
@@ -775,6 +821,11 @@ class TabBar(QTabBar):
                                         window=self._win_id)
             tabbed_browser.wheelEvent(e)
 
+    @pyqtSlot()
+    def tabLayoutChange(self):
+        if self.currentIndex() > -1:
+            QTimer.singleShot(0, self.parent().scroll_tab_bar)
+
 
 @dataclasses.dataclass
 class Layouts: