This applies to the E-mail view in Kontact, the new System Settings layout and the Discover view. Sorry if this bug is mis-filed. When using these applications on Wayland using the KDE Slimbook, they will often distort the display outside of the application. This includes drawing the application view outside of the application's window or drawing it on top of applications that overlap them in the window manager. This does not affect Xorg sessions. When those applications listed above are closed, the distortion ends. I am using mesa on intel graphics and this happens every time.
The same on Arch with KF5.42, Plasma 5.11.95 and amdgpu for Radeon R5
Most likely Qt's broken subsurface handling
No, it's a bug in KWin. *** This bug has been marked as a duplicate of bug 387313 ***