Created attachment 136114 [details]
Test code

SUMMARY

Wayland-based applications which try to hide the mouse cursor by calling wl_pointer_set_cursor() with a NULL surface do not result in the cursor being hidden with KWin. This cursor hiding behavior is explicitly documented in the Wayland protocol [0] and works properly on Mutter and Sway.

[0]: https://github.com/wayland-project/wayland/blob/70f1c83fd9380bf40d904b2806c2af07e688e960/protocol/wayland.xml#L1886


STEPS TO REPRODUCE
1. Build the attached SDL2 sample code
   - On my Fedora and Krypton systems, the following works (assuming SDL2-devel is installed):
     gcc -I/usr/include/SDL2 -lSDL2 -o mouse_hide mouse_hide.c
2. Run the test code in a Wayland Plasma session

OBSERVED RESULT
The cursor is not hidden when it is over the test app's window. The KDE default cursor is displayed instead.


EXPECTED RESULT
The cursor should be hidden when it is over the test app's window.


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: OpenSUSE Krypton
KDE Plasma Version: 5.21.80
KDE Frameworks Version: 5.80.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION

SDL bug report: https://github.com/libsdl-org/SDL/issues/3576

See attached WAYLAND_DEBUG=client log. Particularly the lines where the cursor surface is set to NULL, such as what happens handling the wl_pointer enter event:
[2845237.141] wl_pointer@15.enter(255, wl_surface@21, 3.000000, 321.000000)
[2845237.278]  -> wl_pointer@15.set_cursor(0, nil, 0, 0)

Let me know if there is anything else I can do to help. If you believe SDL's behavior is somehow at fault here, I can work on patches to fix it.