Focus incorrectly managed when using mouse + keyboard

[jessegreenberg]

From https://github.com/phetsims/QA/issues/73

@phet-steele found that this sim is susceptible to https://github.com/phetsims/scenery/issues/713. It is particularly obvious when interacting with the PhET menu. Steps to reproduce buggy behavior:

• Focus "Area" slider by tabbing to it.
• Open the PhET menu with a mouse.
• Focus highlight goes away
• Use arrow keys. Observe that the Area slider moves.

[jessegreenberg]

Actually, it looks like for this sim, this is a buggy implementation for the current implementation of handling keyboard + mouse interaction with the PhET menu. When the PhET menu is shown, we have

        // make sure that any previously focused elements no longer have focus
        Display.focusProperty.set( null );

this removes the focus highlight, but does NOT blur the active element.

[jessegreenberg]

@phet-steele I believe this is resolved in master, would you please verify?

[phet-steele]

Verified!