Created attachment 157562 [details]
Screen content corruption

KDE / Wayland seems to reliably get confused about which screen is attached to which output in a multi-GPU setup.

This materializes in bad / wrong / missing invalidation of painting areas, leaving content garbage in place, and also results in "impossible" state where Spectacle as a screenshotter claims part of a very much existing screen as _transparent_.

The attached screenshot shows the broken state; notice
* the red frame I painted in to show the middle screen ("screen #2") in a triple screen setup
* the bottom-right corner of screen #2, which Spectacle sees as transparent - but which it is not
* the corruption elsewhere on screen #2

Now particularly note the "clean" area on screen #2 - the dimensions here are exactly the same as the dimensions of the screen to the left ("screen #3").

AFAICT, this implies that KWin incorrectly uses the dimensions of screen #3 to compute damage / paint / invalidation areas on screen #2. This then leads to garbage artefacts on the (larger) screen #2.

Key to reproducing this probably is, unfortunately, a GPU setup where

* iGPU = Intel
* dGPU = Nvidia

and where

* screen #1 == laptop == Intel (right-most screen)
* screen #2 == HDMI port _connected to Intel_
* screen #3 == DisplayPort port _connected to NVIDIA_

and where I used "KWIN_DRM_DEVICES=/dev/dri/card1:/dev/dri/card0" to force the NVIDIA GPU as the primary GPU (without this forcing, the Intel GPU becomes primary)

There is no internal mux on the notebook; the outputs are hardwired. In that setup, the NVIDIA card draws everything, draws natively on screen #3, and passes data for screen #2 and screen #1 to the Intel GPU for output.

*******

For the sake of disclosure, the screenshot comes from a bit of additional peculiar set of steps - but it highlights much better the exact same effect seen during "normal operations" with the above setup: The damage / paint area when drawing on screen #2 is taken from screen #3.

The peculiarity is physically turning off screen #3 (the left-most screen), and the Thunderbolt docking station apparently keeps some kind of connection alive, which results in screen #3 still being "seen". But again, I see structurally the same corruption if I don't do that - it's just so much more evident with that peculiar step.

I have no idea why I never see corruption on screen #1 or screen #3.