QMainWindow Menu / actions sometimes not displayed when performing long operations

The attached project contains a MainWindow with a QGraphicsView and an action to "copy" the window => create a new instance of it with the initial window as the parent. On some machines copying the window 1-3 times causes the menu to stop showing entirely.

This problem seems to occur at least on physical machines with:

• CentOS 7.9, kde & gnome
• CentOS 8.5, gnome

This has also been tested with ubuntu, win10 and centos on virtual machines but in these environments the problem did not occur.

Steps to reproduce the problem:

1. run the attached project
2. click "Menu" => "copy"
3. while the second window is being created click repeatedly on the menu (skipping this step may cause the problem not to occur)
4. Attempt to interact with menu after the second window appears
5. Repeat (up to 3 times if problem doesn't occur).

Apparently this is a fairly new issue and cannot be reproduced with early 5.12 bug fix versions.