I'm running Plasma 5.21.3-1 and kdevelop 5.6.2-3 on Arch Linux.
It is hard to put into words what happens when I launch kdevelop. In the following image there is a screenshot of all the desktop (otherwise it would be impossible to spot the problem)



As you can see it is like the desktop is split into two parts and the right one is covering the kdevelop windows. Moving the window around there is no way to make it appear properly. Switching virtual desktop back and fort even weirder effects take places:



I remark that this happen only with kdevelop.
I really do not know what is happening. I even try to build kdevelop and the problem is the same.
Do you have any suggestion?

I've seen a similar bug report recently but without screenshots.

Does the problem persist if you switch compositing off (Shift+Alt+F12 by default)?

Does anything change if you select a different Qt theme, e.g. QtCurve or Fusion?