[CSS Shadow Parts] Bad style sharing between sibling elements with different part attributes

[Justin Fagnani]

The ::part() selector released in 13.1, and as currently implemented in TP and nightly, is selecting the wrong elements when parts are nested in any container element in a shadow root.

Given this DOM:

<!-- This div is necessary to trigger the bug -->
<div>
  <span part="one">Should be Blue</span> | 
  <span part="two">Should be Red</span>
</div>

And these selectors:

my-element::part(one) {
  color: blue;
}
my-element::part(two) {
  color: red;
}

The text spans should be blue then red as described. Instead they are both blue.

If the <div> is removed, or if the second <span> is changed to a different tag, the colors are correct.

JS Bin: https://jsbin.com/bucejul/3/edit

[Serhii Kulykov]

Setting style attribute, e.g. display, or direction attribute on part="two", also forces the red color to apply.

[Radar WebKit Bug Importer]

<rdar://problem/61547528>

[Emilio Cobos Álvarez (:emilio)]

That smells a lot like broken style sharing.

Firefox had a somewhat similar bug in fact: https://bugzilla.mozilla.org/show_bug.cgi?id=1604989

[Antti Koivisto]

Yeah, bet that's the problem. Those spans are obvious style sharing candidates if you forget about 'part'.

[Antti Koivisto]

Created attachment 396079 [details]
patch

[Antti Koivisto]

Created attachment 396082 [details]
patch

[Daniel Bates]

Comment on attachment 396082 [details]
patch

Make sense to me,

[EWS]

Committed r259877: <https://trac.webkit.org/changeset/259877>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 396082 [details].

[Justin Fagnani]

Thanks for the really quick turnaround!

[Justin Fagnani]

A question about the release process for bug fixes like this: will this go out in a patch release on 13.1, or the next big release? ie, will we have to tell developers to use a ::part() polyfill until 13.2?

[Ryosuke Niwa]

(In reply to Justin Fagnani from comment #10)
> A question about the release process for bug fixes like this: will this go
> out in a patch release on 13.1, or the next big release? ie, will we have to
> tell developers to use a ::part() polyfill until 13.2?

It’s unlikely that a bug fix like this makes it to a software update unless there is a major site that’s been actively affected.

[Antti Koivisto]

FYI, this fix is in the current macOS 10.15.6/iOS 13.6 public beta.