Bug 433540

Summary: Wayland cursor hiding via setting NULL cursor surface is broken
Product: [Plasma] kwin Reporter: Cameron Gutman <cameron.gutman>
Component: inputAssignee: KWin default assignee <kwin-bugs-null>
Status: RESOLVED NOT A BUG    
Severity: normal CC: robinoger1207
Priority: NOR    
Version First Reported In: 5.21.1   
Target Milestone: ---   
Platform: Other   
OS: Linux   
Latest Commit: Version Fixed In:
Sentry Crash Report:
Attachments: Test code
WAYLAND_DEBUG=client log

Description Cameron Gutman 2021-02-24 17:55:41 UTC 
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.
Comment 1 Cameron Gutman 2021-02-24 17:56:22 UTC 
Created attachment 136115 [details]
WAYLAND_DEBUG=client log
Comment 2 Vlad Zahorodnii 2021-02-25 08:58:27 UTC 
> [2845237.278]  -> wl_pointer@15.set_cursor(0, nil, 0, 0)

KWin checks the enter event serial, please make sure that SDL passes the right serial to set_cursor request.
Comment 3 Vlad Zahorodnii 2021-02-25 09:01:03 UTC 
I'm closing this bug report as a valid enter event serial must be passed to the wl_pointer.set_cursor request. But SDL applications pass a serial of 0.
Comment 4 Cameron Gutman 2021-02-26 03:09:22 UTC 
Aha! I was staring at these logs and never noticed the missing serial. Thanks for your help.

This is now fixed in SDL - https://github.com/libsdl-org/SDL/commit/8c5b7af2d2f317c42c5437aa4b4dc7c3f5a06057
Comment 5 Vlad Zahorodnii 2021-03-01 07:05:53 UTC 
*** Bug 433756 has been marked as a duplicate of this bug. ***