When a sheet's rightmost compound carries :first-child, :last-child,
or :only-child but no other class/tag/id/attr feature, the sheet
add/remove path can target the boundary children directly instead of
falling back to a whole-subtree invalidation.
The narrowing in extend_style_sheet_invalidation_set_with_style_rule
deliberately only kicks in when the structural pseudo is the only
feature in the rightmost compound. A selector like `.foo:first-child`
keeps targeting `.foo` rather than every first-child in the
document.
Adds matchers for :first-child / :last-child / :only-child in
InvalidationSetMatcher so the targeted walk can recognize them on
each candidate element.
Stylesheet add/remove previously fell back to a whole-subtree
invalidation whenever a sheet's rightmost compound carried one of
these pseudo-classes. They each match a small, knowable set of
elements at any moment, so the invalidation walk can target them
directly:
- :host matches one element per shadow root and :root matches the
html element. Mark them targetable.
- :hover, :focus, :focus-visible, :focus-within, :active, and
:target all match at most a handful of elements at a time. Mark
them targetable and add matchers in InvalidationSetMatcher that
consult Document::hovered_node, focused_area, target_element,
and the element's own focused/active state.
Element.cpp still contained the CSS logic for deciding whether an
invalidation set references features present on an element. Move that
matcher into CSS::Invalidation::InvalidationSetMatcher.
The helper uses Element's public API for classes, id, attributes,
pseudo-class state, and removed-attribute tracking. This keeps Element
focused on DOM state while CSS::Invalidation owns selector feature
matching.
