Fix focus mode on boards

What does this MR do and why?

The change in !72168 (merged) broke Focus mode button on boards.

This keeps that fix and brings back fullscreen button functionality.

Also restores the QA selector removed in !72289 (merged) (see discussion)

Screenshots or screen recordings

Screen_Recording_2021-10-15_at_12.54.28_am

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Load board
2. Click fullscreen button

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.