Weird, is this in the web? If so, try downloading and lmk if it still happens. You can also send a screenshot to the bug reports channel in our discord and we can take a closer look.
I think it might have to do something with firefox. It does this with other unity web games too. It doesn't happen on chrome. It's as if going full screen and click "selects" the game the same way you highlight text with your cursor.
