Add workaround for the "split page" Qt bug (QTBUG-68224)

Fixes #3920

qutebrowser/browser/webengine/webenginetab.py

@@ -1057,6 +1057,22 @@ class WebEngineTab(browsertab.AbstractTab):
     @pyqtSlot(usertypes.NavigationRequest)
     def _on_navigation_request(self, navigation):
         super()._on_navigation_request(navigation)
+
+        if qtutils.version_check('5.11.0', exact=True, compiled=False):
+            # WORKAROUND for https://bugreports.qt.io/browse/QTBUG-68224
+            layout = self._widget.layout()
+            count = layout.count()
+            if count > 1:
+                for i in range(count):
+                    item = layout.itemAt(i)
+                    if item is None:
+                        continue
+                    widget = item.widget()
+                    if widget is not self._widget.focusProxy():
+                        log.webview.debug("Removing widget {} (QTBUG-68224)"
+                                          .format(widget))
+                        layout.removeWidget(widget)
+
         if not navigation.accepted or not navigation.is_main_frame:
             return