If plasmashell is run with QSG_RENDER_LOOP=basic, plasmashell will occasionally redraw some elements of the task manager using the wrong textures. For example, it will redraw an application's task bar entry with a different application's window preview as a background. Taking a screenshot of the issue is not possible, as a complete repaint apparently fixes the glitches. Reproducible: Always Steps to Reproduce: 1. Add a script with the contents "export QSG_RENDER_LOOP=basic" to autostart pre-KDE. 2. Run plasmashell, open a few windows, hover your cursor over the task bar entries 3. Note the glitches The system this was discovered on runs nvidia 349.16 and qt 5.4.1. The issue does NOT occur when QSG_RENDER_LOOP is not set to "basic"
Some sort of issue with the tooltip delegate pre-reparenting maybe ... CC'ing some folks.
My suggestion is: don't set that env variable. On mesa drivers that's the default and it doesn't glitch, on NVIDIA driver threaded rendering is the default. Threaded is better, be happy that it works.
If I don't set that env variable, I hit this bug https://bugs.kde.org/show_bug.cgi?id=347237 100% CPU usage while copying a file isn't really something I'm very fond of.
I see, that's of course a problem and might also explain the glitches you see: optimus. It's not completely unlikely that there is some copying going on. Could you please provide the output of: qdbus org.kde.KWin /KWin supportInformation and glxinfo That could help us get some ideas about your graphics.
Created attachment 92664 [details] qdbus org.kde.KWin /KWin supportInformation
Created attachment 92665 [details] glxinfo
sorry, output doesn't make me any smarter :-(
I'll probably end up waiting for nvidia to finally fix vsync for optimus then, which would make using this option obsolete. Unfortunately, nvidia has historically been slow at fixing anything related to optimus.
This issue might soon become less important since nvidia is working on fixing Optimus/PRIME vsync[1], and with that essentially leave no reason to set this env var on nvidia drivers as far as I can tell. [1]: https://devtalk.nvidia.com/default/topic/775691/linux/vsync-issue-nvidia-prime-ux32vd-with-gt620-m-/post/4715487/post/4715487/post/4715487/#4715487
Let us know if this is still an issue with newer versions of Qt such as the impending Qt 5.6.