REL-2111: fix disappearing panel issue

This is a bug in which a `JPanel`'s content changes but doesn't refresh the view correctly.  Admittedly I've never understood how to do this correctly and cannot seem to parse the differences between all the methods that are apparently related to this issue: `validate`, `revalidate`, `invalidate`, `repaint`, etc.  The change here appears to work fine though.

The bug was introduced when trying to minimize the series of validating/repainting steps and apparently mistakenly testing with a binary that didn't match the code I was looking at.