pedant typography alert.
May I ask - why isn’t there a AUTO mode for text line-height? The usual default for typography is 20% from font-size, so 16px defaults to 19.2px, 14px to 16.8px, etc… doing it manually all the time is kind of annoying.
Did a search and haven’t gotten results, but I find it weird that by version 6 I’m the first to request this… 
You’re actually the first to request it, at least as far as I can remember. It’s a good idea though- I’ll see what I can do about adding it.
Since I was just digging into this code today- setting the line height to 0 is the same as “auto” for Acorn. Maybe I should update the UI to reflect that.
I think that’d be okay. It’s not “0” so that might be confusing to users expecting to see a line-height, but it can work fine. On the odd-occasion the baseline doesn’t reset, and I had it happen once trying out your suggestion of 0-size, but I can’t get it to happen again…so perhaps a user error, but if there’s a way to snap it to Auto or just add a checkbox and disable or set the line-height to the actual line height, I admit I’d prefer that more. I come from a art/DTP background, and now I do UI, so I’m always checking numbers to make sure what I’m showing is what I produce.