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Created attachment 1152 [details]
Sample code

Hey,

I have just stumbled upon some hard-to-spot issue (apparently X11 specific), sometimes reproduced with the attached source file (but not always). It can be summarized as follows: Right after a call to SDL_GetWindowSurface is done, any call to SDL_PollEvent (and/or some other event-related function) can mark the window's surface "invalid" due to a resize event, even when you don't expect it.

Here is how I could reproduce this on Ubuntu 12.04 (for x86-64), using "GNOME Classic (No effects)" and "Unity", although it appears to be reproduced more often while "GNOME Classic (No effects)" is being used:

- Create a non-fullscreen window with some dimensions, say 800x600. Furthermore obtain the window's surface using SDL_GetWindowSurface.
- Do some things.
- Resize window to fit the desktop's dimensions and then toggle fullscreen on. Again update the window's surface.
- Do some more things.
- Toggle fullscreen off and then resize the window to different dimensions, like 800x400. As before, update the surface.
- Check the event queue, ignoring any window resize event.
- Try doing even more things, including any update to the window's surface.

The problem is that, at least sometimes, the following things seem to occur:
- When fullscreen is toggled off, the window's dimensions are restored to 800x600 for a short while.
- These change to 800x400 right afterwards.
- An up-to-date window surface is obtained for now.
- A few "delayed" X11 window events arrive, including two resize events: One resize to 800x600 and another one to 800x400.
- Due to the first resize (if not another one), the surface is marked "invalid" (because the window's dimensions have just been set to 800x400 by the program).

I guess that for now, what can be done is telling a programmer using SDL 2.0 the following:
- Use an alternative to SDL_GetWindowSurface (one of the renderers) if possible.
- If the window surface is used (with SDL_GetWindowSurface), always look for a window resize event, even when you don't expect it. When such an event arrives, SDL_GetWindowSurface should be called in order to obtain an up-to-date surface.
- Alternatively, try updating the surface whenever a call to SDL_UpdateWindowSurface or SDL_UpdateWindowSurfaceRects fails.
- It further worths to look for all events, since two consecutive resize events (if not more) may arrive (like desktop -> 800x600 -> 800x400).