Probably since https://codereview.qt-project.org/c/qt/qtbase/+/636265 by mmutz and richard QDockWidget sends visibilityChanged signal on destruction.

This can lead to crashes in user code when a main widget is supposed to be reacting to changed visibility and e.g. hide/show/modify other data members of same widget.

This is seen in Okular auto tests as part of the tests for Qt Next.

There is also a quite small example in

https://invent.kde.org/sune/windows-test-thingie/-/blob/34f78089876756d1fe662fd7e589da901ba3d6d8/main.cpp