Bug 469227

SUMMARY
Got two screens, one connected via DP (priority 2, left) and one connected via HDMI (priority 1, i.e. "primary", right).
My panel is relatively close to the standard panel layout, except that I use dashboard as a start menu instead of kickoff.
When I start up, everything is working as expected all the time, i.e. nothing goes wrong until I turn off the screens.
If I turn off the screens and then turn them on again, it depends on the order of switching them on whether the positioning of several things is correct or not.

Variant 2:
1. DP (priority 2) on
2. HDMI (prioriy 1) on
Result: correct positioning

Variant 2:
1. HDMI (prioriy 1) on
2. DP (priority 2) on
Result: incorrect positioning

I found two ways of fixing this. Either the screens are switched off and then switched on again according to variant 1. Or the primary screen switch in the kcm is switched back and forth between the two screens.

correct positioning:
Dashboard shows up on the (primary) screen where the panel is.
Tooltips (e.g. task manager, or the notification popup, when you click on the bell) show up at their correct positon.
All context menus show up at the correct positions.

incorrect positioning:
Dashboard shows up on the priority 2 screen.
Tooltips show up on the primary screen, but are aligned to the left screen edge (the one between the two screens).
Some context menus show the same behavior as the tooltips: right click on clock, systray items and empty panel area
right click on task manager items shows the context menu at the correct position, as does it for any other context menu (e.g. in some app like konsole or firefox).
I did not observe any other apps/widgets to misbehave.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 5.27.4
KDE Plasma Version: 5.27.4
KDE Frameworks Version: 5.105.0
Qt Version: 5.15.9

ADDITIONAL INFORMATION
It doesn't seem to happen every time I use variant 2, but in the clear majority of my tries.
It could depend on how quickly I proceed with the second screen, but I'm not sure yet.
If it would be helpful, I might try to investigate this specifically.

Since variant 2 is the way I normally switch on my screens after turning them off, I'm about 90% sure that this bug didn't happen in the first versions of 5.27. Unfortunately, I don't have a quick way to test this specifically without major effort. If it's clear in which git repo I have to look, I could try to bisect the issue.
There are two bug reports that could be related.
bug 468613 reports a wrong positioning of the task manager tooltips, but in this case they do end up on a wrong screen, so I'm not sure if it's the same thing.
bug 383067 I reported years ago, where the dashboard also appeared on the wrong screen. But in this case it was only the dashboard and I actually reported it as working before 5.27, so it might just be the same symptom.

I tested on both X11 and wayland and at least the dashboard thing happens on both. For the tooltips I'm not 100% sure, at least in the few tries I did, they didn't appear wrongly positioned.