If you happen to wonder why #Compositing is broken under #KDE using the #X11 graphics platform, check your ~/.config/kwinrc file
Under the section [Compositing], make sure that the platform interface is set to `glx`:
GLPlatformInterface=glx
Of course making note of the current value in case this breaks things. There is perhaps a GUI but I couldn't find it quick enough and stopped looking.
I realised Compositor was disabled when I saw more black bars in the menus of some apps.
And yes, this also fixes the black bar issue I had with Firefox yesterday, but there I still like having the titlebar anyway.