In a multi-monitor setup clicking on a system icon of an application bring up the window on the screen that last had the focus. This is very annoying when you want to see applications always on a certain screen, and no, making kwin to put on that is not a solution, as you might want to move the app to a different screen for a while and expect to pop up where it was left last time. Reproducible with any app that can be minimized to systray.
X11 or Wayland?
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X11.
Does the problem go away if you either use uncheck System Settings > Window Behavior > Window Management > Advanced > Allow KDE apps to remember the positions of their main windows? Also, what if you use Wayland?
No, that setting doesn't change the behavior. As for Wayland cannot say, I use X11 since forever and never tried to set up wayland.
Here is an example how it goes (test case app Ruqola): - open it on screen 1 - minimize to tray (close the window) - click on the tray icon: it is shown on screen 1. - move to screen 2 - close the window (minimize to tray) - click the tray icon: it is shown again on screen 1 Now move to screen 2, but minimize the window, don't close. Click the tray icon: it is shown where it was on screen 2. Same can be reproduced with KMail or Amarok when tray icon is enabled, but I cannot reproduce e.g with Charm (charmtimetracker).
FTR, Charm is a Qt only app. And I tested with Teamviewer, which is again not a KDE app, that works fine too. So the problem seems to be related to KDE applications.
Can you please try on Wayland? There shouldn't be any real setup necessary; just change the session type to Wayland in the login screen when you log in.
If that would be that simple... :) No, just using Plasma (Wayland) in the login screen does not work, as with this system (for whatever reason, I never debugged so far) it doesn't work, I get back to the login screen.
Still present with X11/Nvidia drivers Plasma 5.26.2
With my current config (first monitor scaled down from 4K with xrandr) it is even worse, the restored window appears partially on the first monitor, partially on the second, no matter where it was when it was closed.
On X11 it's up to the app to remember its window placement while it's "minimized" in the system tray. If you still have this issue, please report it to the affected apps