Fix #3602 (Can't close Quick Docs via Esc after click in left-hand content)

Fix bug #3602 -- Make non-scrolling parts of content focusable so focus stays in widget (where we listen for Esc) rather than bubbling up to some random part of CM that no one's listening to.

This was one of the dangling issues we never got an answer on in pull #3496, but @adrocknaphobia mentioned today it felt like a bug and I'm inclined to agree.