SDL_SyncWindow can time out under normal conditions with a slow computer, with X11 backend

[slime73]

Some context: I'm updating my app to use SDL3, things have mostly gone fine on other platforms but I'm trying to figure out several failures in the app's automated test suite (run on Github Actions CI) on Linux, when the tests validate that window state is consistent after some operations like minimize/maximize/restore. My code uses SDL_SyncWindow to try to make sure that state is consistent.

What I have discovered so far is that, with X11 in the CI test, something like this times out in the last SyncWindow after SetFullscreen(false):

SDL_SetWindowFullscreen(window, true);
SDL_SyncWindow(window);

SDL_SetWindowFullscreen(window, false);
SDL_SyncWindow(window);

The actual code that's reproducing it for me isn't identical to that. I don't have a standalone repro project, but you can see the SyncWindow failure being reported here https://github.com/slime73/love-experiments/actions/runs/11374433695/job/31643430814

I tried increasing the timeout duration from 100ms to 3000ms but it didn't seem to stop that issue, so I wonder if something is actually getting stuck?