Adam Colvin cf67db5f0d LibWeb: Only match ::part() in direct child shadow trees ...

When matching ::part(), only consider shadow trees whose host lies in
the same style scope as the rule. Use MatchContext::rule_shadow_root,
which is already set when collecting ::part() rules, as that scope.

For cross-scope rules, accept a candidate shadow root only if its
host's containing_shadow_root() matches the rule's shadow root: the
rule's scope is the document when rule_shadow_root is null, or that
shadow root when the rule comes from its stylesheet.

The one exception is :host::part(): the rule and the part live in the
same shadow root, so its host is outside the rule's scope and the
comparison would never pass. Allow the check to be skipped for this
case by setting MatchContext::for_host_part_matching when the compound
selector contains :host (directly or inside :is(), which is how CSS
nesting resolves &::part() inside a :host rule).

Run that :host pre-scan only when rule_shadow_root is set. Document
rules never use the same-shadow-root exception; scanning would
otherwise set for_host_part_matching whenever :host appears in the
compound and break cross-scope ::part() matching (e.g. multiple-scopes).

Previously the engine walked all ancestor shadow roots without this
cross-scope check.

2026-03-30 16:47:34 +01:00

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