Plasma sometimes silently permanently switches to a software renderer.  I'm running TW 20240328, Nvidia 550.54, KDE6, X11.

The software renderer is buggy and causes problems with the panel and tray and maybe other things as well.  The panel was being drawn incorrectly, especially the tray.  Clicking on draw items just made the drawing worse. Sometimes all the tray icons were being drawn on top of the launcher-icon. Rebooting had no effect.  

There was a brief flash of a dialog at some point with a warning about software rendering (definitely had the word software, not sure about the rest of the wording).  That was a red-flag, because, on error, KDE5 sometimes does switch to software rendering, but it has a proper dialog warning, and it has a config setting to reenable the GL renderer.  

I couldn't find anything in settings, so I identified files that had changed by comparing with backups. I to diff .config/kdeglobals and found the way to reverse the switch is to edit ~/.config/kdeglobals and remove:

    [QtQuickRendererSettings]
    SceneGraphBackend=software


I think occurs is after plasmashell core dumps and then is attempts to restart more than some threshold number of times (3?).  While attempting to restart it logged:

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Mar 30 10:50:35 kosmos1.gentoo.co.nz plasmashell[7233]: QRhiGles2: Context is lost.
Mar 30 10:50:35 kosmos1.gentoo.co.nz plasmashell[7233]: Graphics device lost, cleaning up scenegraph and releasing RHI
Mar 30 10:50:36 kosmos1.gentoo.co.nz plasmashell[7233]: QRhiGles2: Context is lost.
Mar 30 10:50:36 kosmos1.gentoo.co.nz plasmashell[7233]: Failed to create RHI (backend 2)
Mar 30 10:50:36 kosmos1.gentoo.co.nz plasmashell[7233]: Failed to create QRhi on the render thread; scenegraph is not functional
Mar 30 10:50:37 kosmos1.gentoo.co.nz plasmashell[7233]: Open GL context could not be created
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I suspect plasmashell core dumping had to do with me switching to a console (alt-ctrl-f1), but it doesn't always occur, so it may be timing related.

Anyway, I think a proper dialog should with an OK button should warn that the software renderer is active.  I think there should be a setting that switches back to hardware rendering.  Or the dialog should at lease detail which file to edit and what to change.