Bug 433561

Created attachment 136129 [details]
Video of the issue

SUMMARY
Since Plasma 5.21 I am experiencing this weird bug in which the usable space on my screens is completely random and produces unwanted results. To be more specific, I do as follows:
- I log in
- I attach the second screen (due to bug 429266: if I leave it plugged in before log-in I get the issues described in the bug report)
- the second screen is activated. In the settings it is placed as directly on top of the main screen
- I open any program and it is placed in a messed-up position
The last element makes me think that somehow the usable space of the main screen is overlapping that of the second screen and this leads to the issues. See attached video for reference. Please also note that the menu opens in the middle of the screen, instead of the top left corner.
The only way I found to solve this issue is to manually disable the screen in the settings, then re-enable it and re-place it on top of the main one (a fact that reinforces my idea that something weird is happening with the placement of the screens).

STEPS TO REPRODUCE
1. Attach an external screen
2. Open any app

OBSERVED RESULT
The app opens up in a weird position, with limitations into how an where it can be moved.

EXPECTED RESULT
Everything works normally, so the apps open in sane places and can be moved around freely.

SOFTWARE/OS VERSIONS
Linux: KDE Neon
KDE Plasma Version: 5.21.1
KDE Frameworks Version: 5.79.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
I was able to move the window in the video by pressing "alt", but then it wouldn't move, nor it could be resized.