Bug 509217

SUMMARY
I've got a dual-monitor setup, with the primary display at a 3440x1440 resolution and the second at 1920x1080, with a different wallpaper image on each screen, both set with Positioning = Scaled.  KDE normally starts up with the wallpapers properly sized for each monitor, but sometimes after returning from a fullscreen application, the wallpaper on the larger screen is scaled down to 1920x1080, and placed only in the top-left corner of that screen.  When I enter the Wallpaper settings and select a new image or Positioning type, the image remains confined to the top-leftmost 1920x1080 of the larger screen.

STEPS TO REPRODUCE
1. Configure a dual-monitor setup with different screen resolutions.
2. Set up a desktop background that is scaled across the larger screen.
3. Shift in and out of fullscreen mode in applications (e.g. a Steam game) on the both screens until, after returning from fullscreen on the larger screen, its wallpaper is now confined to only the top-left portion of the screen.
4. Enter the Plasma Wallpaper settings (e.g. by right-clicking on the desktop and choosing "Configure Desktop and Wallpaper")
5. Experiment with different wallpaper images (of varying resolution) and different Positioning choices.  Observe that all wallpapers for the larger screen are confined to the top-left quadrant, within a rectangle whose size corresponds to the resolution of the smaller screen.
6.  Normal behavior (i.e. wallpaper able to fill the entire screen resolution) can be restored by either restarting the KDE session, or by launching a fullscreen application on the larger screen, shifting it to a resolution smaller than the full screen resolution, and then exiting fullscreen mode, which restores the original screen resolution and apparently triggers a refresh of the resolution detected by the Wallpaper utility.

OBSERVED RESULT
Wallpaper is occasionally confined to an area smaller than the actual resolution of that screen.

EXPECTED RESULT
The entire screen area should always be available for wallpaper or other screen backgrounds (e.g. solid-color fill).

SOFTWARE/OS VERSIONS
Linux/KDE Plasma:
KDE Plasma Version: 5.27.12
KDE Frameworks Version: 5.116.0
Qt Version: 5.15.17
Graphics Platform: Wayland
Backend: Kernel 6.15.4 amdgpu driver, Mesa 25.2.2

ADDITIONAL INFORMATION
Similar issues were reported in #452308 (reported as fixed in 5.25) and #459407 (reported as fixed in 5.26), but it's clearly still happening in 5.27, so I don't believe the underlying cause has been fully addressed in the 5.x series.

#487717 describes another issue regarding confused screen resolutions, triggered by turning on the monitors in a certain sequence, which does not appear to be a factor in my case (both monitors were on and active continuously from normal behavior through the emergence of the fault and the eventual resolution).  The #487717 comments show that it was initially reported fixed in 6.1, then reopened after it was confirmed still present in 6.1.0, 6.1.2, and 6.1.3.  The last comment states "A number of multi-monitor robustness changes have made it into Plasma 6.2, so for the moment, let's call it fixed until someone is able to reproduce it", which is plausible but not a terribly data-driven approach to defect tracking.

I think the real takeaway is that these multi-monitor bugs depend on complex interactions, may have multiple trigger paths, and should not be prematurely closed without extensive testing.