Bug 260716 – adoptedStyleSheets (ObservableArray) has non-writable length

• Component: DOM
• Status: RESOLVED FIXED

[Nolan Lawson]

Created attachment 467431 [details]
Repro screenshot of Chrome, Firefox, and Safari Technology Preview

The `adoptedStyleSheets` ObservableArray should have a `length` property that is writable. (This is consistent with how regular JavaScript arrays work.)

However, in Safari Technology Preview (Safari 17.0, WebKit 18617.1.4.3), the `length` property is non-writable, and writing to it does not affect the underlying array.

Minimal repro:

    const sheet = new CSSStyleSheet()
    sheet.replaceSync('div { color: red }')
    document.adoptedStyleSheets.push(sheet)

    document.adoptedStyleSheets.length = 0 // this should empty the array

In the attached screenshot, you can see that this leads to an observable difference between Chrome and Firefox (text is black) vs Safari Technology Preview (text is red). Here is a CodePen repro [1].

Additionally, the `writable` property on the property descriptor returns `false` in Safari but `true` in Chrome and Firefox. (Repro: [2])

     Object.getOwnPropertyDescriptor(document.adoptedStyleSheets, 'length').writable

In the spec [3], there is an explicit example [4] of setting `length` to 0 to demonstrate that the array is mutable.

[1]: https://codepen.io/nolanlawson-the-selector/pen/bGOVLre
[2]: https://codepen.io/nolanlawson-the-selector/pen/rNoOJyp?editors=1010
[3]: https://webidl.spec.whatwg.org/#idl-observable-array
[4]: https://webidl.spec.whatwg.org/#example-observable-array

[Nolan Lawson]

Created attachment 467432 [details]
Minimal repro HTML

[Alexey Proskuryakov]

Thank you for the report! This reproduces with Safari 16.5.1 too, so not a recent regression.

[Karl Dubost]

Maybe depends on Bug 238281

[Radar WebKit Bug Importer]

<rdar://problem/114822538>

[Chris Dumez]

Pull request: https://github.com/WebKit/WebKit/pull/17350

[EWS]

Committed 267594@main (8e072e6f9bed): <https://commits.webkit.org/267594@main>

Reviewed commits have been landed. Closing PR #17350 and removing active labels.