If a composite widget is put behind one of it's contained children via QWidget::setTabOrder, then our logic might replace the composite widget with the contained child. In that case, we'd end up with a broken tab chain, possibly resulting in incomplete clean-ups and triggering asserts when shutting down the UI. Handle this by stopping the last-child searching logic at the respective other widget, and by not allowing both widgets to be the same. Augment test case, and apply some minor refactoring to the code. Pick-to: 6.5 6.2 Change-Id: I7d97dfa85315b6c01daa283253025d94a1727048 Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>