Fix type mismatch for reflected FrozenArray<T>?

[gmvrpw]

Currently, the "cached attr-associated elements" variable has a list of elements type but a null value can be assigned.

Browsers work logically as possible and ignore this strange behavior, which can be easily verified:

myDiv = document.createElement('div')
// <div>​</div>​
// cached attr-associated elements = [] 
// cached attr-associated elements object = null
myDiv.setAttribute('aria-describedby', 'id')
myDiv.ariaDescribedByElements
// [], but elements (result of **get the attr-associated elements**) is [] (myDiv is not connected),
// so "contents of elements ([]) is equal to the contents of this's cached attr-associated elements ([])" is true,
// so cached attr-associated elements object (null) must be returned.

• At least two implementers are interested (and none opposed):
• [Tests] custom-elements/element-internals-aria-element-reflection.html (as stated below)
• Implementation bugs are filed: Not needed
• 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.

---

/acknowledgements.html ( diff )
/common-dom-interfaces.html ( diff )

[zcorpan]

Thanks for this PR!

It's not editorial because there is a change in what is required. As such, there should be tests. Please put back the PR template. If browsers already pass the tests the implementer interest checkbox can be checked.

[gmvrpw]

Thanks for clarifying. I'll return with the tests.

[annevk]

This is tested by custom-elements/element-internals-aria-element-reflection.html though only a single test.

[annevk]

But yeah, please restore the PR template as well as we want to do our due diligence here. cc @twilco

[annevk]

I think this is correct. I think we could also have returned early for elements being null, but that would keep around a garbage cached value for longer, which isn't exactly clean.

[gmvrpw]

Description has been restored 🤖
It's a shame I saw these messages too late, so here's my implementation of the tests. Let me know if it's useless.