If I alt-tab out, I cannot tab back. There is also a 50-50 chance that the game starts tabbed out. Maybe there is some problem with GMS2's full screen mode.
Otherwise, pretty clever. I fought the cutest ghost ever at darksouls bossmusic. It's ability to randomly rearrange the placed pieces, was more of a helpful boon than an issue. The transformer randomly changing my pieces was far more painful. Found no bugs, and it seems to be able to go for a while.