---
id: render-re-renderable
name: Make Root.render() re-renderable
status: pending
depends_on: [mount-with-fibers]
created: 2026-05-18T16:22:57.183371873Z
modified: 2026-05-18T16:22:57.183372316Z
scope: moderate
risk: medium
impact: component
level: implementation
---

# Description

Make `Root.render()` support being called multiple times. Currently, calling `render()` twice creates two independent instance trees appended alongside each other. After this task, the second `render()` call reconciles against the stored fiber tree: it compares the new UNode tree to the existing fiber tree and applies property-only updates (structural reconciliation is Phase 2).

This task also addresses the `render()` → `mount()` naming question from host-config.md Open Question 1: consider whether renaming `render()` to `mount()` for the first call makes the semantic clearer.

For Phase 1 (before key-based reconciliation), `render()` handles:
- First call: mounts the tree and builds the fiber tree
- Subsequent calls: reconciles props positionally (same structure assumed), calls `prepareUpdate`/`commitUpdate` for changed props

## Acceptance Criteria

- [ ] `Root.render(node)` on first call mounts and builds fiber tree (existing behavior)
- [ ] `Root.render(node)` on second call reconciles props against existing fiber tree
- [ ] Second `render()` does NOT create duplicate instances
- [ ] Positional children matching: Nth old child → Nth new child
- [ ] If child count differs, excess old children remain (structural changes deferred to Phase 2)
- [ ] `Root.unmount()` still a stub (proper unmount is Phase 3)
- [ ] Existing tests pass
- [ ] New test: calling `render()` twice on same root produces one instance tree with updated props
- [ ] New test: `prepareUpdate` is called for changed props on re-render

## References

- docs/architecture/host-config.md — Known Gaps: The Reconciler Gap, render() is not idempotent
- docs/architecture/reconciler.md — Step 2 (Reconcile Props), Changes to Existing Files
- docs/architecture/host-config.md — Open Question 1 (render → mount rename)

## Notes

> To be filled by implementation agent

## Summary

> To be filled on completion