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.
This change converts `Node::invalidate_style()` (invalidation sets
overload) from eagerly doing tree traversal that marks elements affected
by invalidation set to instead adding "pending invalidation sets" into
`StyleInvalidator`, processing of which is deferred until the next
`update_style()`. By doing that we sometimes substantially reduce amount
of work done performing tree traversal that marks elements for style
recalculation.
Improves performance on Discord, were according to my measurements we
were previously spending 20% of time in style invalidation, but now it's
down to <1%.
This change introduces StyleInvalidator as a preparation for upcoming
change that will make `perform_pending_style_invalidations()` take care
of pending invalidation sets.
