---
id: TASK-004
title: Editor rebuilds full visual model twice per keystroke (O(document))
status: "\U0001F3C1 Done"
assignee: []
created_date: '2026-06-28 18:38'
updated_date: '2026-06-30 00:49'
labels:
  - perf
  - release-1
dependencies: []
priority: low
ordinal: 4000
---

## Description

<!-- SECTION:DESCRIPTION:BEGIN -->
Every InsertRune→followCursor→buildVisual rescans+rewraps all lines; MoveUp/MoveDown call buildVisual then followCursor calls it again (2 full scans/vertical keystroke). Fine for notes, costly for long docs in the writing canvas. Fix: thread one []vrow through move→followCursorWith(rows); longer term cache the visual model, invalidate on edit/resize.
<!-- SECTION:DESCRIPTION:END -->

## Acceptance Criteria
<!-- AC:BEGIN -->
- [x] #1 at most one buildVisual per keystroke
<!-- AC:END -->

## Implementation Plan

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1. RED: add buildCount instrumentation + tests asserting MoveUp/MoveDown/MoveToVisual each call buildVisual at most once per keystroke.
2. GREEN: extract followCursorWith(rows []vrow); have followCursor() = followCursorWith(buildVisual()). Thread the single rows slice from MoveUp/MoveDown/MoveToVisual into followCursorWith instead of re-scanning.
3. Verify suite + vet green; check AC #1.
<!-- SECTION:PLAN:END -->

## Implementation Notes

<!-- SECTION:NOTES:BEGIN -->
Extracted followCursorWith(rows); MoveUp/MoveDown/MoveToVisual now scan once and reuse the rows for scroll-follow (was 2 scans/vertical keystroke). buildCount instrumentation + 3 perf tests assert <=1 buildVisual per move. Suite + vet green.
<!-- SECTION:NOTES:END -->