MS4 takes a long time to close; system slows to a crawl in the meantime; force quit dialog opens

Describe the bug

This problem has ocurred repeatedly over several months. It seems to happen when you leave MS4 open with a score in progress, then use another program such as Mozilla Firefox, then try to close MS4.

To Reproduce

The following may work for you:

1. Open MS4
2. Create a treble staff score.
3. Enter a few notes etc.
4. Open the browser and use it for perhaps ten minutes or more (the length of time may, or may not, be a factor)
5. Click back into MS4, then try to shut the program by clicking on the window cross icon.

RESULT: The program hangs. It may take some time to close. Meanwhile the rest of the system may slow to a crawl.

Expected behavior

MS4 should close right away. The response of the rest of the system should not be affected.

Platform information

OS: Linux Mint 20.1, Arch.: x86_64, MuseScore version (64-bit): 4.0.0-230040304, revision: github-musescore-musescore-eab50c9