Use cached select in selectedness setting algorithm

[josepharhar]

In order to make the selectedness setting algorithm match implementations, this PR makes the selectedness setting algorithm avoid changing the selectedness of option elements which haven't ran their insertion steps yet by checking whether the options have their cached nearest ancestor select element assigned yet or not.

This was discussed here: #11825

• At least two implementers are interested (and none opposed):
  • Chromium
  • …
• Tests are written and can be reviewed and commented upon at:
  • https://wpt.fyi/results/html/semantics/forms/the-select-element/inserted-or-removed.html
• Implementation bugs are filed:
  • Chromium: Already implemented
  • Gecko: …
  • WebKit: …
• Corresponding HTML AAM & ARIA in HTML issues & PRs: Not needed
• MDN issue is filed: Not needed
• The top of this comment includes a clear commit message to use.

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

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/form-elements.html ( diff )

[annevk]

How does this PR relate to #11890? It's getting quite confusing.

[josepharhar]

How does this PR relate to #11890? It's getting quite confusing.

I listed some different ways we can proceed here: #11890 (comment)

[zcorpan]

Is this sufficiently tested by web-platform-tests/wpt#55647 ?

[josepharhar]

I removed the code which does this in chromium in order to find tests which would fail, and I found that html/semantics/forms/the-select-element/inserted-or-removed.html would fail without this logic, specifically with this failure:

This is a testharness.js-based test.
[FAIL] The last selected OPTION should win; Inserted by DOM API
  assert_equals: expected "Second" but got "First"
Harness: the test ran to completion.

I'll add this test to the OP

[josepharhar]

We also have an internal test which would fail without this, which I could move to WPT: https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/web_tests/fast/forms/select/multiple-selected-options-innerHTML.html