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Created attachment 3280 [details]
Patch to fix loose RESIZED events on SIZE_CHANGED

At the bottom of this post is a test program that demonstrates what's happening, at least on Windows 10.

I'm finding that Windows REALLY likes to dump lots of resize events when toggling fullscreen; in particular it sends a resize event as it's resizing the window, before SDL is finished changing the window size. The result is this:

- Windows fires RESIZED
- RESIZED fires SIZE_CHANGED, previous SIZE_CHANGED events cleared
- Previous RESIZED events cleared
- SDL sets window size, fires SIZE_CHANGED event, previous SIZE_CHANGED events cleared but RESIZED events are NOT cleared
- Application polls events, conflicting size events!

On Windows you can actually get a good sense of the problem simply by toggling fullscreen a bunch, no clever setup needed. The way I fixed this is by making it so any SIZE_CHANGED event clears out _all_ of the size events, so whatever the latest size event is the only one in the loop. User resizes will still always get exactly 1 SIZE_CHANGED and 1 RESIZED, but programmatic changes will always get exactly 1 SIZE_CHANGED and 0 RESIZED if they end up overriding any resizes between frames.

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#include <SDL.h>

int main(int argc, char **argv)
{
	SDL_DisplayMode mode;
	SDL_Window *window;
	SDL_Event evt;
	SDL_bool ftoggle = SDL_FALSE;
	Uint8 run = 1;

	SDL_Init(SDL_INIT_VIDEO);
	SDL_GetCurrentDisplayMode(0, &mode);
	window = SDL_CreateWindow(
		"WindowEvent Test",
		SDL_WINDOWPOS_CENTERED,
		SDL_WINDOWPOS_CENTERED,
		800,
		480,
		SDL_WINDOW_SHOWN
	);

	while (run)
	{
		while (SDL_PollEvent(&evt) > 0)
		{
			if (evt.type == SDL_QUIT)
			{
				run = 0;
			}
			else if (evt.type == SDL_KEYDOWN)
			{
				if (ftoggle)
				{
					SDL_SetWindowFullscreen(window, 0);
					SDL_SetWindowSize(window, 800, 480);
				}
				else
				{
					SDL_SetWindowSize(window, mode.w, mode.h);

					/* Just trying to replicate FNA... */
					SDL_Delay(1);

					SDL_SetWindowFullscreen(
						window,
						SDL_WINDOW_FULLSCREEN_DESKTOP
					);
				}
				ftoggle = !ftoggle;
			}
			else if (evt.type == SDL_WINDOWEVENT)
			{
				if (evt.window.event == SDL_WINDOWEVENT_SIZE_CHANGED)
				{
					SDL_Log(
						"%d SIZE CHANGE %d %d",
						SDL_GetTicks(),
						evt.window.data1,
						evt.window.data2
					);
				}
				else if (evt.window.event == SDL_WINDOWEVENT_RESIZED)
				{
					SDL_Log(
						"%d RESIZE %d %d",
						SDL_GetTicks(),
						evt.window.data1,
						evt.window.data2
					);
				}
			}
		}
	}

	SDL_DestroyWindow(window);
	SDL_Quit();
	return 0;
}

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