Fix focus navigation trap when starting point is inside focused area, but before any other focusable areas.

[ltenmoz]

Fixes #12514

• At least two implementers are interested (and none opposed):
  • Gecko
  • …
• Tests are written and can be reviewed and commented upon at:
  • Tentative tests at html/interaction/focus/sequential-focus-navigation-starting-point.tentative.html. I don't think it's possible to make a non-tentative test for this, since the only places in the standard where the starting point is set also run the focusing steps on it. (In practice, all browsers move it on click, which is what the tests uses, but it's not specified.)
• Implementation bugs are filed:
  • Chromium: N/A (sequential focus navigation starting point is not used if there is a focused element)
  • Gecko: (already implementing)
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=316027

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/interaction.html ( diff )