I'm on Linux Mint 18.4 KDE.

For about a week now after something updated I now have this problematic desktop behavior where if I maximize a window then restore it small again the area it had been covering while maximized stays black. I have to hit the "Show Desktop" widget twice to toggle everything to get it back to normal.

Screenshot of it wrong:
https://files.catbox.moe/yh4x7x.png
(That green text is a flashing part of the webpage behind the black)

Screenshot of what it should be:
https://files.catbox.moe/5w6myg.png

bump...

Just changed to a new kernel & it appears to have resolved the condition.

Old:
4.15.0-45

New:
4.15.0-46

Today I upgraded from a kernel where the aforementioned issue wasn't an issue, to a kernel where it once again was a problem.

This is how I solved it without having to switch away from the new 'problematic' kernel:

KDE System Settings > Display and Monitor > Compositor > Rendering backend:
-Change from 'OpenGL 2.0' to 'OpenGL 3.1'
Apply

...To prevent flickering caused by the above:
-Change 'Tearing prevention ("vsync"): to 'Never'
Apply