When I start a snapshot (any snapshot) in fullscreen and press F11 to switch to windowed mode or when I start the game in window mode, switch to fullscreen and back, the window has the standard size and can't be resized anymore. When I start the game in windowed mode, I can resize it, but after switching to fullscreen and back the size is locked.
For full releases it works, but every snapshot has this issue. Very weird.
Ignore the texture pack in the video, it's not even finished yet. I recommend watching it in higher speed, I recorded it on my laptop.