Perf: dispatch id/class attribute mutations through the token index (#150 Tier 2)#158
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…150 Tier 2) Stacks on #157. Closes the regression flagged in that PR's risk section: a jQuery-style `element.id = 'x'` or `classList.add('x')` on an already-inserted element used to be incidentally caught by the old full-doc rescan, but the token-index dispatcher's per-added-subtree walk would miss it. Adds an opt-in `observeAttributes` flag to createSubtreeWatcher. When any subscriber asks for it, the shared MO upgrades to `attributes: true, attributeFilter: ['id', 'class']` — and the router fans attribute mutations only to subscribers that requested them, keeping the existing rules off the hot path. The token-index opts in so its dispatcher re-walks the changed element through the same findTriggeredRules path as a freshly-added subtree. Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>
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Stacks on #157.
Summary
Closes the one behavioral regression flagged in #157's risk section: attribute mutations on already-inserted elements. Before this PR, the token-index dispatcher only walked added subtree roots, so jQuery-style
element.id = 'cookie-banner'orclassList.add('cookie-banner')after insertion was silently missed. The old code (pre-#157) caught these incidentally because every batch ran a full-doc QSA.Changes
lib/subtree-watcher.ts: opt-inobserveAttributes?: booleanoncreateSubtreeWatcher. When any subscriber asks, the sharedMutationObserverupgrades to{ childList, subtree, attributes, attributeFilter: ['id', 'class'] }. The router fans attribute mutations only to subscribers that opted in — other rules don't see them.MO.observe()re-call on the same target.lib/selector-token-index.ts: dispatcher opts in. Attribute targets flow throughfindTriggeredRulesthe same way as added subtree roots.Why opt-in (not blanket)
22 other rules use
createSubtreeWatcherdirectly (pii-redact, secrets-redact, json-ld-sanitize, etc.). Most don't benefit from re-scanning a node just because a class toggled, and pages doing animation-driven class toggling could produce MO storms. The opt-in keeps the existing fast path for those rules intact; only the selector-hide-rule dispatcher pays the attribute-observation cost.What about MO config replacement semantics
Per the DOM spec, calling
observe()on the same MO with the same target updates the registered observer's options in place — no historical record re-emission. We rely on that for both the upgrade and downgrade paths. The router holdsobservingAttributesto make the desired config recomputable from current subscribers.Test plan
bun run test— 1254 / 1254 passbun run typecheckbun run check(biome + eslint)bun run knip<div>is inserted with no id/class, page later setsdiv.id = 'late-id'ordiv.classList.add('late-class'), the rule's removeEntirely path fires on the second drain.🤖 Generated with Claude Code