Where to modify the logic that calculates the height of a collapsible fieldset? I'm not sure if this is a theme issue, but I continually get my collapsible fieldsets being 1-2 scanlines too short, causing scroll bars to appear.

This project has heavy use of FAPI, sometimes with collapsible fieldsets inside other collapsible fieldsets, and the vertical scroll bars are eating too much page real estate.

I'm using an unmodified version of Fusion Starter theme, if that helps. And, I'm not a themer, but can usually get by. 

I would be perfectly happy modifying the PHP or JavaScript code doing this calculation, if I just knew where it is.
Are you sure it can't be solved by CSS? Try adding some padding.