[Feature Request] composable for searchable trees

Origin

• vuetify#22983 - draft for improvements for VTreeview

High level: tree structures (treeview, datatables with group-by, lists) might benefit from headless composable driving search «» opened interaction while also supporting open-all.

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Filtered reveal for nested trees (useNestedReveal + Treeview filter prop)

A searchable tree needs its matches to become visible — which means opening every collapsed ancestor of each match. Doing this naively (recompute openedIds from the match set on every change) breaks real UX in ways that are non-obvious until you hit them. This was discovered while patching Vuetify's VTreeview. PR attempts to introduce open.ts that should eventually be replaced by v0-native utility.

createNested already ships the mechanics — reveal(ids) (opens ancestors only), open/close, openedIds, getAncestors, isLeaf, and a registration-driven openAll. What is missing is the filter-reveal session policy: the rules for how open state should evolve as a query comes and goes, without fighting the user.

createFilter already produces the match set. Nothing wires the two together.

The policy (behavior spec)

Given a reactive set of matched ids (from createFilter) and a NestedContext:

1. reveal, don't dump.

• for each match, open nodes path (reveal)
• open the matched node itself.

2. additive, not authoritative.

• merge reveal targets into openedIds. A branch the user collapses mid-search stays collapsed (until a new query makes it match again).

3. remember only what the search opened.

• track the ids this session actually opened (those not already open when revealed)

4. on clear, undo only the temporarily revealed nodes — except what user opened deliberately mid-search
5. accept live tree changes. — as matches appear/disappear (async load, edits), reveal nodes and leafs that match

Proposed API

export function useNestedReveal (
  nested: NestedContext,
  matchedIds: MaybeRefOrGetter<Iterable<ID>>,   // e.g. from createFilter → ids
  options?: { active?: MaybeRefOrGetter<boolean> }, // default: derive from matchedIds emptiness
): void

• No return value; it drives nested.openedIds as a side effect while active.
• active false (query cleared) triggers the rule-4 cleanup exactly once.
• Pure policy over reveal/open/close/openedIds/getAncestors/getPath/
  isLeaf. Ships with createNested conceptually; keep it a separate export so
  non-tree consumers of createNested don't pay for it.

Component surface

Add a filter/query prop to Treeview.Root (mirrors openAll/reveal which already live there). Internally: createFilter → matched ids → useNestedReveal(nested, ids). Visibility of non-matches is a separate render concern (a matched(id) predicate on the
context for the item to consult), independent of open state — keep the two decoupled.

Test checklist (from Vuetify's VTreeview tests)

• reveal a match nested under collapsed ancestors (both eager + lazy registration)
• collapse a revealed ancestor mid-search → stays collapsed when search/query is cleared
• clear removes only session-opened nodes
• clear keeps a branch the user opened mid-search and its adopted ancestors
• match added tree modes while active is revealed
• openAll + collapsed nodes do not re-open accidentally