Viewport not updating

[numpad]

Steps to reproduce:

1. Start the game, the resolution will be correct at (X*Y)
2. Make the window fullscreen, resolution will also be correct at (X2*Y2)
3. Bring the window to the resolution (X*Y) again by leaving fullscreen, the viewport/resolution will be incorrect and only render to about half the window.

This does not seem to happen when resizing the window by changing its size, only when the resize happens instantaneously (like fullscreen, or switching between tiled and floating in bspwm)