perf(synthetic-shadow): optimize recursivelySetShadowResolver

[nolanlawson]

Details

Optimizes the recursivelySetShadowResolver function to avoid calling childNodes and to use firstChild/nextSibling instead.

Browsers represent the DOM as a linked list under the hood, so this is faster.

This improves the dom-ss-slot-create-container-5k-this-change benchmark by 4-9%, and possibly some other synthetic-shadow benchmarks as well.

The reason this is important is because we set KEY__SHADOW_STATIC on the top level of a static fragment, which then causes us to recurse through the entire DOM structure to set the KEY__SHADOW_RESOLVER. (This is used for the legacy isNodeShadowed/isNodeFromTemplate as well as for the patched MutationObserver.) If we have a lot of static fragments, or if they are very deep, then we can spend a lot of time recursing.

Long-term, we should probably try to avoid this recursion entirely and just do the lookup on-demand when MutationObserver/isNodeShadowed is called: #4089

Does this pull request introduce a breaking change?

• 😮‍💨 No, it does not introduce a breaking change.

Does this pull request introduce an observable change?

• 🤞 No, it does not introduce an observable change.