1739682 - Bare `::part(foo)` CSS selector selects parts inside shadow roots

Status: NEW

(Core :: CSS Parsing and Computation, defect, P3)

Description

[Nolan Lawson]

Attachment: Minimal repro

User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:94.0) Gecko/20100101 Firefox/94.0

Steps to reproduce:

1. Go to https://bl.ocks.org/nolanlawson/raw/cbc40307e15f99be6e084ac0fffb59be/

Actual results:

The <span> is blue.

Expected results:

The <span> should be blue, because it's being targeted with the CSS selector ::part(foo), which should not work.

According to a comment in this WebKit bug, the fact that it appears blue in Firefox should be considered a bug. WebKit and Chrome do not have this behavior, although WebKit just added support for :host::part(foo).

From a web developer POV, I'd say that ::part(foo) is problematic because it's ambiguous in cases where a custom element's shadow root contains another custom element with its own part that has the same name.

Comment 1

[Nolan Lawson]

The <span> should be blue

Sorry, this is a typo. I meant "the <span> should not be blue."

Comment 2

[BugBot [:suhaib / :marco/ :calixte]]

Set release status flags based on info from the regressing bug 1624968

Comment 3

[titob]

Can this be marked as a bug instead of wontfix? Please see: https://github.com/w3c/csswg-drafts/issues/9981 Thanks

Comment 4

[Emilio Cobos Álvarez (:emilio)]

The bug is still open. It was sent to wontfix for firefox 95/96, because we're not fixing it in those versions.

It's a bit tricky to fix but probably not too terrible... We either fix the matching of ::part() to make sure that if we're on the same tree we have a featureless host selector to the left, or fix the rule collector to not look at those rules.