Reveal cursor on panel expand

@peterflynn Simple fix for issue #3003, and (nearly) identical to closed and outdated pull request #3931.
Seems like this comment could be more precise -- what does "maintaining the distance between the cursor and the bottom" mean given that the cursor is _below_ the bottom at the time?  Clearly we don't want to maintain _that_ distance...  It sounds like what we actually mean is something like "restoring the distance between the cursor and the bottom that existed before any bottom panels were shown"?