Sorry to hear that, what system and browser are you using? When I was testing it on windows 10 and Firefox it also just closed the tab on certain spots, I'm trying to find the cause of it.
We should've added more versions for compatibility as well, something to keep in mind for the future.
But thank you for checking us out regardless ^~^
UPDATE: Ctrl+W closes the current tab, but Chrome blocks that shortcut if you play in fullscreen mode. On Firefox it looks like it will close the tab regardless.