When inheriting custom-property data from a parent element, we were
copying the parent's full CustomPropertyData regardless of whether
each property was registered with `inherits: false`. That caused
non-inheriting registered properties to leak from the parent,
contrary to the @property spec.
Wrap the parent-side lookup so we strip any custom property whose
registration says it should not inherit, and only build a fresh
CustomPropertyData when at least one property was actually filtered.
Key the filtered view's cache on both the destination document's
identity and its custom-property registration generation. The
generation counter is local to each document, so a subtree adopted
into another document (or queried via getComputedStyle from another
window) could otherwise pick up a cached view computed under an
unrelated registration set and silently skip non-inheriting filtering
in the new document.
Replace per-element OrderedHashMap storage for custom properties with
a RefCounted chain (CustomPropertyData) that enables structural
sharing. Each chain node stores only the properties declared directly
on its element, with a parent pointer to the inherited chain.
Elements that don't override any custom properties share the parent's
data directly (just a RefPtr copy). During cascade, only entries that
actually differ from the parent are stored in own_values - the rest
are inherited through the chain. During var() resolution, resolved
values are compared against the parent's and matching entries are
dropped, enabling further sharing.
The chain uses a depth limit (max 32) with flattening, plus
absorption of small parent nodes (threshold 8) to keep lookups fast.
This reduces custom property memory from ~79 MB to ~5.7 MB on
cloudflare.com.
