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SUMMARY
Many fullscreen windows, such as games, "wiggle" when losing and regaining focus. This seems to be because when the window loses focus, it's moved above the taskbar for one frame before being repositioned to the bottom of the screen. This doesn't occur with all fullscreen windows; it consistently occurs with some games (Eve Online and Noita, for example) running under Wine, whereas many other fullscreen applications (Firefox, mpv, Jellyfin, the game Factorio) do not trigger this.

A workaround for the issue is to un-fullscreen the window (right click in taskbar > More > untick Fullscreen), manually move it down so the top left corner of the window is in the top left corner of the screen, and then re-fullscreen it.

STEPS TO REPRODUCE
1. Open an affected fullscreen application (e.g. the game Noita)
2. Bring up the taskbar (e.g. by pressing meta to open the application launcher, or by alt-tabbing to another window)

OBSERVED RESULT
The affected window is moved above the taskbar for one frame before being moved back down.

EXPECTED RESULT
The affected window doesn't move and instead just layers underneath the taskbar.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Fedora 40, kernel 6.8.8-300, Wayland session
(available in About System)
KDE Plasma Version: 6.0.4 
KDE Frameworks Version: 6.1.0
Qt Version: 6.7.0

ADDITIONAL INFORMATION