Bug 294961 – After click tabindex element in shadowDom, cannot navigation by Tab key

• Status: NEW
• Component: DOM

[Woody Lee]

DOM:
```
<my-element>
  <div tabindex="0">
    <div id="inner">Inner content</div>
  </div>
<my-element>
```

Demo: https://codepen.io/Woody-lxf/pen/WbvLLyo

1. Visit the demo.
2. Click the #inner div.
3. Using the keyboard Tab key to navigate.

Then the focus element is always the tabindex div, and cannot navigate to others.
But the Tab navigation works well after clicking on other positions.

[Woody Lee]

Tried to change the delegatesFocus property, but it still exists.

[Radar WebKit Bug Importer]

<rdar://problem/154838788>