UNDERTALE was updated on Steam to fix bugs and increase compatibility.
However, now the Mac version may crash every time you hold ESC to quit. It has no gameplay effect, but doesn't look good. This is a known bug inherent to Gamemaker (software I use) that can't be fixed until it's updated. Will fix it as soon as possible.
If the update causes your game to NOT work or you like bugs, revert to old version: Right-click UNDERTALE in Library-->Properties -->Betas -->old_version_100
By the way, this update doesn't fix Steam Client Beta's latest update issues with UNDERTALE. If UNDERTALE's suddenly stopped launching, try reverting to the normal version of the Steam Client.