Created attachment 133270 [details]
Black border around context menu in Dolphin

SUMMARY

Since the 5.20 release I'm seeing black borders around some menus.

STEPS TO REPRODUCE
1. Open Dolphin.
2. Right click at the Menu bar.

OBSERVED RESULT
The menu has a rectangular black border.

EXPECTED RESULT
The menu has a usual grey border.

SOFTWARE/OS VERSIONS
KDE Plasma Version: 5.20.2
KDE Frameworks Version: 5.75.0
Qt Version: 5.15.1
Kernel Version: 5.8.18-1-MANJARO
Graphics Processor: Mesa Intel® HD Graphics 530

ADDITIONAL INFORMATION
Without compositing everything looks the same.
Some other places displaying the same effect:
- the hamburger menu in sidebar-style System Settings;
- the menubar menu in kwrite, gwenview and other apps where it's applicable;