Customizer Menus: Accessibility: create new menu, focus goes to the preview area in Firefox

[designsimply]

/hat tip afercia for reporting this at ​https://github.com/voldemortensen/menu-customizer/issues/127

Reported by Michelle deYoung from the accessibility testers group. See: ​https://make.wordpress.org/accessibility/2015/06/12/accessibility-usertest-menu-customizer/

Issue: Once a new menu is created, the keyboard focus is moved to the content area.

Initially I couldn't reproduce this, she didn't mention the specific browser she used so I was using Chrome. When you press Enter or Spacebar on the "Create Menu" button, in Chrome all is fine, focus is moved to the new panel.
By the way with Firefox something weird happens: after you press "Create Menu", press the tab key 3 times and you're in the preview iframe. Looking closely here's what happens:

1. press "Create Menu" either with Enter or Spacebar, doesn't matter
2. tab once: focus is on the "Collapse" link
3. tab again: focus is on the iframe (Firefox gives focus to iframes)
4. tab again: focus is on the first focusable element in the preview

I think (not sure) this has something to do with timings: probably Firefox is not able to give focus to a panel while the animation is running and the element not fully visible yet.

Worth noting a complete focus loss like this one would make the Customizer hardly usable by keyboard users.

[valendesigns]

I'll look into a patch sometime this next week.

[valendesigns]

I cannot reproduce this issue, could you please verify it still exists.

[afercia]

Yeah cannot reproduce anymore, maybe magically auto-fixed with latest changes.

[obenland]

@designsimply, could you double-check on your end and close as appropriate?

[ocean90]

Confirmed, can't reproduce anymore.