Reveal cursor on panel expand

@peterflynn Simple fix for issue #3003, and (nearly) identical to closed and outdated pull request #3931.
Do we always want to scroll any time the cursor is "below the fold"?  It seems like if I scroll way higher than the cursor, switch to another file, then switch back, this will make the view snap back to the cursor.  Ideally we'd only touch the scroll pos when the cursor _used to_ be visible, and has just become invisible only due to a panel being shown.