Avoid repeated :has() child-list scheduling when pending data already
covers every concrete feature bucket used by :has() selectors in a style
scope. Featureless-sensitive scopes record child-list mutations
conservatively instead.
That conservative path avoids scanning mutation subtrees for concrete
features when invalidation cannot rely on them anyway. When loading
the Intel ISA PDF in pdf.js, instrumented subtree feature-collection
visits at about 40k style invalidations dropped from around 71k to 1.6k,
saving ~650ms of main thread time on my Linux machine. :^)
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