Before this change, whenever element's attributes changed, we would add
a flag to "pending invalidation", indicating that all descendants whose
style uses CSS custom properties needed to be recomputed. This resulted
in severe overinvalidation, because we would run invalidation regardless
of whether any custom property on affected element actually changed.
This change takes another approach, and now we decide whether
descendant's style needs to be recomputed based on whether ancestor's
style recomputation results in a change of custom properties, though
this approach adds a little overhead to style computation as now we have
to compare old vs new hashmap of custom properties.
This brings substantial improvement on discord and x.com where, before
this change, advantage of using invalidation sets was lost and we had
to recompute all descendants, because almost all of them use custom
properties.
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