STEPS TO REPRODUCE 1. Have a few maximized apps open 2. Alt+tab between them OBSERVED RESULT Occasionally i see 1 or 2 frames of the desktop background, now that it's dark it's super distracting EXPECTED RESULT Just switch between the apps, don't show me the background SOFTWARE/OS VERSIONS Linux/KDE Plasma: Archlinux KDE Plasma Version: 5.24.0 X11 KDE Frameworks Version: 5.90.0 Qt Version: 5.12.kde Here a video recorded, in the video it's much harder to appreciate than in real life, i guess my camera is not very good and youtube potentially also dropped some frames https://www.youtube.com/watch?v=j_HLc7XcD8w but if you see a screenshot i managed to capture of that video https://i.imgur.com/Kcj8QWn.png on the top right you can see that red/orange-ish part belonging to the background.
Qt 5.15.kde obviously, sorry for the mental-slip
Can you post the output of `qdbus org.kde.KWin /KWin supportInformation`? Can't reproduce it myself.
On this computer i can reproduce it *VERY* easily https://ghostbin.com/RULuW/raw (alt-tabbing and alt+shift-tabbing between konsole and firefox will make it happen say 30% of the time). On my other (faster/newer laptop https://ghostbin.com/EFiEo/raw ) I can see it sometimes, but much less often, maybe it's a driver/CPU speed issue? I'm going to try with a much slower one now. Also make sure you use the Plasma 5.24 background, it being so distinctively black and red in places defenitely helps realizing "this should not be here"
Does this happen with other task switchers, e.g. breeze? Does kwin also print any warnings in the logs?
Yes, it also happens with the breeze switcher. This is what i get from a "clean" kwin_x11 --replace https://ghostbin.com/49Iw5/raw I don't get "new" debug/warnings when the issue happens.
As a baseline i disabled the compositor (Alt+Shift+F12) and this problem doesn't happen anymore
Cannot reproduce at all on Wayland.
I'm not sure it's a regression, this may have been happening for a while, just the new wallpaper is much more intense so seeing it is much more distracting. On Matrix we tracked it to the "highlight window" effect Vlad said if you switch from window A to B, the highlight window effect (enabled by "show selected window" in system settings) will start two animations A: opacity goes from 1 to 0 B: opacity goes from 0 to 1 so desktop background can sneak through And suggested that maybe the highlight window effect could start 0 -> 1 animation first and delay 1 -> 0 animation I'll try to see if i can make that happen and test it myself, though the code in https://invent.kde.org/plasma/kwin/-/blob/master/src/effects/highlightwindow/highlightwindow.cpp is not as easy to understand as i'd like ^_^
After discussing on Matrix, parallel cross fading will always result in this, needs to be sequential, but I don't have enough understanding of the code to be able to work on that, so someone else needs to tackle this. kbroulik: if you have two items ontop of each other with opacity < 1 you will always see through ahiemstra: combining opacity is more like averaging their values rather than adding them up generally, for cross fade animations what you want to do is have the original image at opacity 1 until the end and fade the new image from 0 to 1 on top of that