Fix Expander appearance on Windows.

On Windows, the frame for the expander buttons was not being shown at all.
This cleans that up. Hopefully, things are still good on other platforms.

Change-Id: Ieef06be1df7a735d82b051c8cef1b685f5c5aadf
Merge topic 'refactor_panels' into next

c3b1044 Fix Expander appearance on Windows.