This only happens when pressing C-n inside a drawing in normal (not maximized) mode.

The reason is complicated. First, the baseline:

• When pressing C-o: run.keychord: starty is not nil run.keychord: current drawing is nil Drawing.keychord C-o: doesn't set current drawing plan_draw (only happens in normal mode): clears starty in all lines Drawing.update: all good ✓

• When pressing C-n: run.keychord: starty is not nil run.keychord: current drawing is nil Drawing.keychord C-n: sets current drawing <=== plan_draw: clears starty in all lines Drawing.update: BOOM

Ugh.

When is starty set? Drawing.draw. This is why I can clean it up with abandon. But: frame = events -> update -> draw

• plan_draw clears starty while processing events
• Drawing.update blows up in here <===
• Drawing.draw is about to set starty

So one possible interpretation here is that this is an unfortunate interaction of two special-cases:

• draw should never mutate state, but it's just the most convenient place to track starty
• keystrokes never set current drawing – except rare operations like naming and moving points

So the current prescription: any code called from within update needs to protect against starty being nil.

Ugh X-(