Reveal cursor on panel expand

@peterflynn Simple fix for issue #3003, and (nearly) identical to closed and outdated pull request #3931.
@peterflynn @busykai I think using `setSize()` to call the function with a delta would be the cleanest solution. We wouldn't need to compute anything in PanelManager, it'll still only be passing the flag to the handler which will then pass it to `setSize()`.

Using a delta would also eliminate any guesswork about previous editor size.