SUMMARY
The line numbers panel becomes transparent and flickers for a short moment when saving or reloading a file under Wayland.

STEPS TO REPRODUCE
1. Log in to a Wayland session
2. Configure Kate to show line numbers
3. Open some file
5. Trigger save (Ctrl+S) or reload (Ctrl+F5)

OBSERVED RESULT
The line numbers panel gets transparent for a short time. The window below is shining through. There might be visual artifacts like a short flickering.

EXPECTED RESULT
There should be no visual glitches in the line numbers area. The window below should never be visible.

SOFTWARE/OS VERSIONS
Operating System: KDE neon 5.21
KDE Plasma Version: 5.21.4
KDE Frameworks Version: 5.81.0
Qt Version: 5.15.2
Kernel Version: 5.4.0-72-generic
OS Type: 64-bit
Graphics Platform: Wayland
Processors: 4 × Intel® Core™ i5-3570K CPU @ 3.40GHz
Memory: 7.2 GiB of RAM
Graphics Processor: Mesa DRI Intel® HD Graphics 4000

ADDITIONAL INFORMATION
This issue is Wayland only and applies to all katepart-based text editors (kate, kwrite, kdevelop).
The described behaviour is annoying for people who tend to save files very often.