Bug: Alt-Tabbing causes FSO to freeze intermittently

[LuytenFS]

Alt-Tabbing in the latest Linux nightly build causes a loss of window focus across windowed, borderless, and fullscreen modes. Mouse and keyboard inputs fail to register upon returning, leaving the game frozen or non-responsive in both gameplay and menus. The screen occasionally goes black, and regaining focus requires cycling Alt-Tab which often doesn't even fully capture focus.

2026-08-15_09-38-13.mp4

[wookieejedi]

Just to check, were you able to ID a specific nightly this bug starts on? As in, Aug 14th build works just fine? Thanks!

[LuytenFS]

Excuse the late reply I was busy, the build in the video was August 15th yes. Knossos also does not display to me any August 14th builds however, so I tested August 8th which is the earliest i could find near August 14th, which the bug was still present. The issue was actually only absent in July 23th build (07/23) after some rounding, the builds after that have the bug present.

[Goober5000]

Claude says this is caused by the SDL3 port, #7220
issue7710_investigation.md

cc @notimaginative

[notimaginative]

1. Fullscreen went from desktop-fullscreen to exclusive mode-set fullscreen

I was unable to reproduce the SDL2 behavior in a functional way. It would cause rendering issues or outright crash due to a resolution mismatch (on Mac at least). Setting window size is a request, not an absolute, and until the FSO graphics code can be made to work properly at an other-than-expected window size I'm not sure there is a simple fix for this that works across platforms, OpenGL, and Vulkan. Ideally the rendering resolution would be separate from the window size.

Using exclusive mode wasn't a choice, it's what it took to get it working. I would have avoided it if I had the option. So if Claude has a suggestion that works I'd be glad to give it a try.

However, the bug behavior in question looks to happen the same in all window modes, so this specific change is unlikely to be the culprit.

2. SDL3 prefers native Wayland; SDL2 ran through XWayland

Certainly a possibility. Testing with X11 instead would be a good idea just to rule Wayland out as the culprit. However all of the bugs mentioned by Claude had been fixed by the time the first SDL3 test builds showed up last year. There is also a PR waiting in the prebuilt repo that updates to the newest stable SDL3 version (at the time the PR was made anyway), so perhaps it has additional fixes.

But I did test pretty thoroughly under Wayland and don't recall having this issue. I could have just missed it, or there is something else contributing to the bug as well.

3. Minor state-tracking change (gr_activated starts as true)

That was to fix an issue with the FSO window being created, minimizing, then restoring at startup. I'm not sure why SDL2 builds didn't have the same issue and I never really tried so find out since the simple change in question fixed it without appearing to break anything else.

--

My theory is that the bug has something to do with fAppActive, or at least with the related behavior. I'd previously tracked several force feedback issues to that behavior and I don't think it was ever really resolved to my satisfaction. I don't recall making a change which would have caused this, but perhaps a combination of small things is triggering it.

I'll try to do some testing this weekend and see if I can figure anything out. (Number 5 in the suggested fixes looks like a good place to start)

In the meantime @LuytenFS , if you could please test with X11, either by setting SDL_VIDEO_DRIVER=x11 env variable or actually running under X11, that would help rule out a general Wayland+SDL3 issue as the cause. Also maybe give one of the older SDL3 test builds on Knossos a try and see if they have the same problem, just in case the bug is related to SDL3 + another code change.

[Goober5000]

I asked Claude to respond and here's what he wrote. He retracted the fullscreen theory and had a few suggestions:
issue7710_response.md