If you enter/leave fullscreen mode a bunch of times, you will notice that the Minecraft window will begin to expand horizontally and shrink vertically.
Steps to reproduce:

1. Open Minecraft.
2. Enter/leave fullscreen mode by either pressing F11 key or "Fullscreen" button in "Video Settings" a bunch of times. (The bug occurs even when you press it once, however it might be easier to reproduce by changing to fullscreen a bunch of times)

Observed results:

Minecraft window will begin to expand horizontally and shrink vertically.

Expected results:

Minecraft window should be the same size, as before it was fullscreened.