When triggering an overlay, tab order should be dynamically managed.

[Westbrook]

Expected Behaviour

You trigger an overlay and either you are sent into it by default, or tab takes you into it.

Actual Behaviour

You could get lost being the DOM is appended to the and intervening content could steal the focus as if part of the natural tab order.

Reproduce Scenario (including but not limited to)

• Visit https://opensource.adobe.com/spectrum-web-components
• type into the search field
• when the results display, try to tab into them 😱

[Westbrook]

fixed by #728