Run VirtualThought's size-release cleanup only on unmount

[raineorshine]

Follow-up to #4969, which fixed useDelayedAutofocus latching an unmounted ref in a cleanup that React re-runs on every dependency change. This is the same mistake in VirtualThought, found by auditing the codebase for it.

Problem

VirtualThought releases its tracked size by calling onResize({ height: null }) in an effect cleanup, annotated:

// these should be memoized and not change for the life of the component, so this is effectively componentWillUnmount
[crossContextualKey, onResize, id],

onResize is not stable. TreeNode memoizes it as useCallback(props => setSize({ ...props, cliff }), [cliff, setSize]), and cliff changes whenever a thought's position in the tree changes. React runs the cleanup before every such re-run, so a thought releases its size while still mounted.

Instrumenting the cleanup and adding a sibling (which changes the preceding thought's cliff):

=== adding sibling to a1 (changes cliff of a1) ===
CLEANUP-FIRED |_fHZM6U06I_ej stillInDOM= true
  removeSize |_fHZM6U06I_ej
  setSize    |_fHZM6U06I_ej 20

Impact

No user-visible bug today. updateSize's dependencies are a superset of this effect's, so its body re-adds the entry in the same passive-effect flush — the trace above shows the removeSize/setSize pair. What this PR removes is the churn (an in-place delete on the sizes state object, then a rebuild) and a live trap in the size-tracking path: any future change that breaks the superset relationship, or that makes the removal observable, turns this into a layout bug.

Solution

Route the call through the existing useFreshCallback so the effect's dependency is stable and the cleanup runs only on unmount, while still invoking the latest onResize. The comment now describes why the indirection is needed instead of asserting a stability that does not hold.

Verification

• Re-ran the instrumented trace: the mid-life fire is gone, and all four thoughts still fire on teardown (stillInDOM= false).
• yarn lint:tsc
• eslint src/components/VirtualThought.tsx
• Full unit suite: 1731 passed, 0 failed.

No regression test is included — the current behavior is self-healing, so there is nothing externally observable to assert without instrumenting internals.

🤖 Generated with Claude Code