Been having this problem despite several installations and keyboard checks.
It only occurs when I'm building or something similar. I'll press the alt key to go into super shift, put down a couple of bricks and the screen turns black, and Blockland takes about five minutes and turns into full screen mode. I go back to normal windowed mode and it works fine, but I'll start building again and it takes me back to full screen. Back and forth matter, too. I've tried just staying in full screen, but it'd move me back to windowed mode.
I kept checking my advanced controls and normal controls for any type of shortcut that'd bring me into fullscreen that I've been accidentally pressing, but I see nothing.
Is there something I'm doing that leads me into full screen? I'm using the default control scheme, with a laptop keyboard.