options.unmount ancestor walk makes whole-tree unmount O(hook components × depth) #5217

We're the ReactLynx team (lynx-family/lynx-stack) — ReactLynx ships a Preact fork as its renderer. Migrating from 10.x to 11 regressed page teardown.

46ddd2f9 added this walk to options.unmount in hooks, to find the nearest still-mounted ancestor:

let errorParent = vnode._parent;
while (errorParent && !(errorParent._component && errorParent._component._parentDom)) {
  errorParent = errorParent._parent;
}

During a full unmount there is no such ancestor. unmount() clears _parentDom on the way down:

render(null, container)

  unmount(App)          App._parentDom = null
    unmount(Wrapper)      Wrapper._parentDom = null
      unmount(Leaf)         ← walk runs here, every ancestor already null

So the walk always reaches the root and yields null. Subtree unmounts are fine — the parent above the removed root is still mounted, so it stops on step one.

Benchmarks

Our destroyBackground suite splits on one thing: does the fixture use hooks? (full CodSpeed report on our PR, Simulation mode = instruction counts, not timing noise.)

rc.1 made everything ~5% faster. Only the hook fixture regressed — matching where the walk sits, inside if (c && c.__hooks). Instrumenting it during a real page destroy: { calls: 1000, iters: 3000 }, i.e. 1000 components × 3 levels, exactly the tree depth.

Repro

lynx-family/internal-preact@c61f5b1 — a test/browser test with a fixed leaf count, varying only depth:

Every component in a full unmount resolves to the same errorParent, so it could presumably be computed once per pass. We don't know why the parentVNode argument was dropped, so we're reporting rather than proposing a patch.