I tested with Xournal++ in KDE Neon User running Plasma 6.3.4, but I'm not able to reproduce the hang. I'd like to ask a few questions to help narrow down the cause.

Does this happen on your system with X11 apps other than SDL3?

If you open the Crashed Process Viewer, do you see anything with a timestamp from around the time your system hung?

If there aren't crashes around that time, please try getting logs of the hang by running these two commands:

echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
sudo gdb -pid $(pidof kwin_wayland) -batch -ex "set logging file kwin_wayland.gdb" -ex "set logging enabled on" -ex "continue" -ex "thread apply all backtrace" -ex "quit"

After you reboot, you should have a log file "kwin_wayland.gdb" in your home directory

Then start SDL3 and cause the bug to happen.

I did have plasmashell freeze back in February when moving a FreeTube window around, but that was with plasma Version 6.2.4, and the problem stopped happening in later plasma versions

(In reply to TraceyC from comment #1)
> I tested with Xournal++ in KDE Neon User running Plasma 6.3.4, but I'm not
> able to reproduce the hang. I'd like to ask a few questions to help narrow
> down the cause.
> 
> Does this happen on your system with X11 apps other than SDL3?
> 
> If you open the Crashed Process Viewer, do you see anything with a timestamp
> from around the time your system hung?
> 
> If there aren't crashes around that time, please try getting logs of the
> hang by running these two commands:
> 
> echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
> sudo gdb -pid $(pidof kwin_wayland) -batch -ex "set logging file
> kwin_wayland.gdb" -ex "set logging enabled on" -ex "continue" -ex "thread
> apply all backtrace" -ex "quit"
> 
> After you reboot, you should have a log file "kwin_wayland.gdb" in your home
> directory
> 
> Then start SDL3 and cause the bug to happen.

So when it happens it leave no errors in the log. If I move the window then resize to trigger it and then click on any X11 window or spawn the right click on any other app the session hangs. I can hear it when it trigger since it pins a core at 100%.

[New LWP 2464]
[New LWP 2463]
[New LWP 2462]
[New LWP 2461]
[New LWP 1306]
[New LWP 1297]
[New LWP 1296]
[New LWP 1268]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/nix/store/rmy663w9p7xb202rcln4jjzmvivznmz8-glibc-2.40-66/lib/libthread_db.so.1".
0x00007f157570d34e in ppoll () from /nix/store/rmy663w9p7xb202rcln4jjzmvivznmz8-glibc-2.40-66/lib/libc.so.6
warning: File "/nix/store/vi9n40g3814x3f7sqnc3jvmrhr4n3ij9-glib-2.82.5/lib/libgobject-2.0.so.0.8200.5-gdb.py" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load:/nix/store/ik84lbv5jvjm1xxvdl8mhg52ry3xycvm-gcc-14-20241116-lib".
To enable execution of this file add
        add-auto-load-safe-path /nix/store/vi9n40g3814x3f7sqnc3jvmrhr4n3ij9-glib-2.82.5/lib/libgobject-2.0.so.0.8200.5-gdb.py
line to your configuration file "/root/.config/gdb/gdbinit".
To completely disable this security protection add
        set auto-load safe-path /
line to your configuration file "/root/.config/gdb/gdbinit".
For more information about this security protection see the
"Auto-loading safe path" section in the GDB manual.  E.g., run from the shell:
        info "(gdb)Auto-loading safe path"
warning: File "/nix/store/vi9n40g3814x3f7sqnc3jvmrhr4n3ij9-glib-2.82.5/lib/libglib-2.0.so.0.8200.5-gdb.py" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load:/nix/store/ik84lbv5jvjm1xxvdl8mhg52ry3xycvm-gcc-14-20241116-lib".
[New Thread 0x7f151f6f26c0 (LWP 2751)]
[New Thread 0x7f151eef16c0 (LWP 2752)]
[New Thread 0x7f151e6f06c0 (LWP 2754)]
[New Thread 0x7f151dea66c0 (LWP 2758)]
[Detaching after vfork from child process 2761]
[Thread 0x7f151e6f06c0 (LWP 2754) exited]
[Thread 0x7f151dea66c0 (LWP 2758) exited]
[Thread 0x7f151f6f26c0 (LWP 2751) exited]
[Thread 0x7f1539fa86c0 (LWP 2464) exited]
[Thread 0x7f151eef16c0 (LWP 2752) exited]
[Thread 0x7f15397a76c0 (LWP 2463) exited]
[Thread 0x7f1533fff6c0 (LWP 2462) exited]
[Thread 0x7f153a7a96c0 (LWP 2461) exited]
[New Thread 0x7f153a7a96c0 (LWP 2776)]
[New Thread 0x7f151eef16c0 (LWP 2777)]
[New Thread 0x7f1539fa86c0 (LWP 2778)]
[New Thread 0x7f15397a76c0 (LWP 2779)]
[Thread 0x7f1539fa86c0 (LWP 2778) exited]
[Thread 0x7f15397a76c0 (LWP 2779) exited]
[Thread 0x7f151eef16c0 (LWP 2777) exited]
[Thread 0x7f153a7a96c0 (LWP 2776) exited]
[New Thread 0x7f15397a76c0 (LWP 3216)]
[New Thread 0x7f1539fa86c0 (LWP 3217)]
[New Thread 0x7f153a7a96c0 (LWP 3218)]
[New Thread 0x7f151eef16c0 (LWP 3219)]

I clicked the close button when I triggered (no response of course) it and I think its [Detaching after vfork from child process 2761].

Thanks for that information. The detail about this being related to CPU usage is helpful.

Can you run a tool like top that will show process usage in a window alongside the xwayland application, and then cause the bug to happen?
Capture the screen with your phone if you need to in order to get a screenshot, and attach that to this report. This will help us narrow down the process that's causing the problem.

Thanks!

🐛🧹 ⚠️ This bug has been in NEEDSINFO status with no change for at least 15 days. Please provide the requested information, then set the bug status to REPORTED. If there is no change for at least 30 days, it will be automatically closed as RESOLVED WORKSFORME.

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🐛🧹 This bug has been in NEEDSINFO status with no change for at least 30 days. Closing as RESOLVED WORKSFORME.