As the title suggests, when switching from fullscreen to windowed mode the maximize button is disabled and is not enabled again until after you restart the game.
EDIT: Windows 8.1 Pro, Single screen, gForce GTX 880m w/4g video ram.
[0.12.6] [kovarex] Maximize button disabled when switching from Fullscreen to Windowed.
Re: [0.12.6] [kovarex] Maximize button disabled when switching from Fullscreen to Windowed.
Thanks for the report, it is fixed for 0.12.7