Preserve focused area across bfcache reactivation

[hjanuschka]

Promotes the existing non-normative note about focused-area preservation to normative prose, and explicitly states that no focus or blur events fire as a side effect of a Document moving into or out of the back/forward cache. This aligns the spec text with the implementer agreement reached in #5878 and matches WebKit's shipping behavior.

Companion issue: #12496

Motivating implementation: Chromium CL https://crrev.com/c/7829940 (https://crbug.com/40199280) is blocked on this clarification.

Checklist

• At least two implementers are interested (and none opposed):
  • WebKit: already implements this behavior.
  • Chromium: implementing in https://crrev.com/c/7829940.
  • Gecko: focus is preserved today; blur/focus event suppression part may need a follow-up.
• Tests are written and can be reviewed and commented upon at:
  • https://github.com/web-platform-tests/wpt/blob/master/html/browsers/browsing-the-web/back-forward-cache/focus.html (existing, from [WPT] BFCache: focus should be kept around BFCache web-platform-tests/wpt#31084).
• Implementation bugs are filed:
  • Chromium: https://crbug.com/40199280
  • Gecko: (TBD)
  • WebKit: n/a (already matches)
• Corresponding HTML AAM & ARIA in HTML issues & PRs: n/a (no element semantics changed).
• MDN issue is filed: (TBD)
• The top of this comment includes a clear commit message to use.

Preserve focused area across bfcache reactivation

Promotes the existing note's preservation statement to normative prose
and clarifies that no focus or blur events fire as a side effect of a
Document moving into or out of the back/forward cache. This aligns the
spec text with the implementer agreement reached in #5878.

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/interaction.html ( diff )