There is one quite strange problem with AlwaysOnTop window, system - Qt5.9.1, Win10 x64, MSVC2017 x64 app.
Let's imagine simple aplication with QMainWindow that contains only one QComboBox. When I set to windows flags Qt::WindowStaysOnTopHint after ui->setupUi(this) in constructor then application window after start and unfold combobox list is like:
but when I switch to another window(s) and back, and again unfold combobox list, then is look like this:
Simply - listbox is hidden behind main form partially or whole depends on its size.
It is not tied on one special machine , I reproduced it on two independent Win10 x64 Pro computers.
If form is not always-on-top, then everything works OK.
Code in main dialog window:
When I unfold listbox of combobox first time it look like:
1. appears listbox
2. part of main form is redrawn over listbox
3. listbox is redrawn again over form,
but after switching to another window and back the third step is omitted. so it stays in state like the second picture.