QMainWindow:.tabifiedDockWidgets() was refactored in Qt 6.6.3, because it returned wrong values. It now loops from 0 to QMainWindowTabBar::count() and uses QMainWindowTabBar::dockAt(), to get the dock widget pointer for an index. When a dock widget is removed from a floating tab, event processing is necessary for the item_list and QMainWindowTabBar::count() to get in sync. This case was overlooked in the refactoring. It can lead to dockAt() being called with an out-of-bounds index, which it asserted. The assertion triggered, when QMainWindow::removeDockWidget() and QMainWindow::tabifiedDockWidgets() were called right after each other, without processing events inbetween. QMainWindowTabBar::dockWidgets() doesn't add nullptr to the list, it returns. Therefore, return nullptr, when dockAt() is called out-of-bounds. Add test functionality in tst_QDockWidget::updateTabBarOnVisibilityChanged(). Fixes: QTBUG-124262 Pick-to: 6.7 6.5 Change-Id: If66084b9f00b4e39f7a620da68df67c735029cf1 Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>