I pushed a commit to git, and what a way to lose an evening. This is actually two-part:

• Builder, apparently, has logic to auto-clean cache whenever a compile error occurs and after all game windows were closed.
  I feel like it wasn’t very successful at it, but maybe I accidentally fixed that.
  I added a pair of checkboxes to settings to control this and a menu button to explicitly clear cache.
• Using forward slashes in paths now breaks caching in GameMaker compiler, which is probably a bug but also I’m not sure if I can complain about it since invoking compiler on your own isn’t documented to begin with.

All in all, this adds to a big pile of examples that GameMaker updates moving quicker also mean that assets and tools just got a whole bunch more expensive to maintain.