kwin_wayland: Windows are often invisible and unusable, triggers session crash
When opening any new window (application, dialog, menu, etc) the window will often never appear and remains in an invisible state. Its process does however exist and is fully responsive. KWin seems to think it's there too, however Plasma doesn't show it in the task manager and alt-tab switching selects something non-existent... for example the drawing and selection order of windows completely breaks when an unrendered dialog is present. If attempting to switch desktops during this time, the session will crash and you're taken back to the login screen: Only way to "disarm" such crashes is to kill the process behind such a window (eg: from KSysGuard)... attempting to open the application again will usually work, though it may take several tries before the window will spawn properly. This issue renders the Plasma Wayland session next to unusable and requires constant user attention to work around hidden windows.
I tried several things which don't seem to prevent the problem: First was a suggestion to boot with the amdgpu.dc=0 parameter which did not affect the issue. I attempted different combinations under System Settings - Hardware - Display and Monitor - Compositor, the scale method / render backend (OpenGL 2.0 or 3.1) / vsync didn't affect it either. I also played with some parameters under the Compositing section of ~/.config/kwinrc but couldn't find proper documentation on what each one does and what in there might help.
Linux openSUSE Tumbleweed x64. Plasma 5.18.4. Mesa 20.0.4. Wayland 1.18 / 1.20. KWayland 5.68. AMD Radeon (TM) R9 390 Series (HAWAII, DRM 3.36.0, 5.6.2-1-default, LLVM 10.0.0). KWin theme is Aurorae, themes in use include: