Clarify that form state restoration does not fire events

[domenic]

(But, it does fire formStateRestoreCallback, since that is explicitly used for synchronizing the custom element's internal state.)

Closes #6853.

• At least two implementers are interested (and none opposed):
  • Nobody fires such events; Chrome tried and ran into some compat issues
• Tests are written and can be reviewed and commented upon at:
  • Hmm, this seems pretty testable. @mfreed7 would you be able to port the following to WPTs? https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/web_tests/fast/forms/range/range-restore-oninput-onchange-event.html + https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/web_tests/fast/forms/file/file-input-change-event-on-back-forward.html
• Implementation bugs are filed:
  • Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=1244603, https://bugs.chromium.org/p/chromium/issues/detail?id=1131234
  • Firefox: N/A
  • Safari: N/A

(See WHATWG Working Mode: Changes for more details.)

---

/browsing-the-web.html ( diff )
/custom-elements.html ( diff )
/input.html ( diff )

[domenic]

I filed https://bugs.chromium.org/p/chromium/issues/detail?id=1266468 on upstreaming some tests since I guess we should not block this clarification PR on that effort.