[aszlig]

There is a "Auto-hints" setting that should highlight the undo button in red once you're in an unwinnable state: https://uncrossy.com/settings.html

[paulnpace]

That option shows up in the Puzzle Solved! dialog box.

#Uncrossy Jan 8 in 18 moves, in 2:23, no hints, no resets, no undos, auto-hints on ⬅⬅⬅⬆⬅⬅⬆⬇⬆⬇⬅⬇⬆⬇⬅⬇ https://uncrossy.com