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This might be an issue with the window manager more so than x11 itself. However, I would suggest trying it with VLC in OpenGL to confirm.
Are you running the recommended drivers for your GPU?
If VLC doesn't do it, check to see how their code differs from yours perhaps?
I have tge same issue on at least two working distros, and had similar issues on previous ones I used. Other programs don't have this issue, only mine, when resizing. I don't know if I should call some extra functions, since most documentation I could get is pretty much "just use some existing solution".
This is a wild guess but you probably need to rerender the framebuffer when you get the resize event. I'd expect most libraries/frameworks/engines would take care of that for you, but if you've rolled your own using the base X11 and OpenGL libraries you might need to set that up. X11 has a lot of strange quirks due to its network centric design.