This is probably just an ACE editor issue but if you use the multicursor feature to, say, unindent a whole bunch of lines at once, and then, say, accidentally backspace too far with 100 cursors so all your lines get concatenated or worse, then you won’t be able to undo that. Because an undo action will be created for every single cursor and the undo buffer isn’t that big.
That’s especially bad in Glitch since you may be kind of screwed unless you exported to GitHub recently. Maybe that’s an argument for an “Export to GitHub” button in addition to the keyboard shortcut (cmd-shift-E)?
Or does ACE give any way to replay the whole history of edits to a file, like Etherpad does?