[MU4 Issue] [macOS] If modal dialog is on second display, menu bar items can become unexpectedly enabled

[cbjeukendrup]

Describe the bug
If you move a modal dialog to a second display, and then click on a menu in the (system) menu bar on the first display (that contains the main window), the actions in that menu are unexpectedly enabled.

To Reproduce
Steps to reproduce the behavior:

1. Open a score in MU4
2. Open a modal dialog, for example File > Parts
3. Move the dialog to the second screen
4. On the first screen, click on the system menu bar to open a menu (it's good that you can open those menus)
5. Note that the commands inside the menus are unexpectedly enabled, as if no dialog is open. This enables you to open another modal dialog at the same time.

Expected behavior
Those menu commands should be disabled when a modal dialog is open, and the enabled-ness should be the same on each screen.

Screenshots
On the first screen:

Schermopname.2022-07-12.om.14.31.52.mov

The result on the second screen:

Platform information

• OS: macOS only, since it involves the system menu bar

Additional context

• The main window itself is properly disabled. It's just the actions in the menu bar.
• On the screen that contains the dialogs, the actions are properly disabled.
• Can reproduce in Dorico (Qt Widgets) too (although in Dorico only one modal dialog can be open at a time, so if you try to open a second dialog, it will appear when the first one is closed)
• Cannot reproduce in MuseScore 3 (Qt Widgets)
• also not in Qt Creator (Qt Widgets)
• also not in MS Word (some other framework, maybe partly native)
• also not in native macOS apps because those use special "window-modal" dialogs that can't be detached from the main window.