Reveal cursor on panel expand

@peterflynn Simple fix for issue #3003, and (nearly) identical to closed and outdated pull request #3931.
The status bar "height" computed using the padding on `$scrollerElement` turns out to be 30px, while the real height is only 26... thus the 4px difference.

However I've noticed that the 30px padding doesn't even come from Brackets CSS but rather from the CodeMirror repo which means we shouldn't be relying on that at all.