diff --git a/HELPERS.md b/HELPERS.md
index db58032..fb573e3 100644
--- a/HELPERS.md
+++ b/HELPERS.md
@@ -797,6 +797,7 @@ Reading a cut's layer tree: order, cache keys and change signatures.
 | `flattenForCanvas` | The layers to draw, bottom first, with folders resolved and hidden branches dropped. |
 | `strokeSig` | A cheap change signature for a layer's strokes, used to invalidate the layer canvas cache without stringifying the whole array. Sound because strokes here are only ever appended or replaced. |
 | `layerSig` | The cache key for one baked layer canvas. Two caches use it and compare their keys against each other, so for a layer that is not boiling both forms must come out byte-identical - otherwise every such layer misses the cache and is redrawn every frame, with no visible symptom. |
+| `appendedAfter` | How many of a layer's strokes a canvas baked under a given signature already holds, when the strokes are that same list with more on the end — so a commit draws only the new ones instead of redrawing the layer. `null` when anything else changed. Leans on the same `rev` invariant the cache already needs. |
 | `flattenLayersInUiOrder` | The layer tree flattened the way the panel shows it, so an index in the list means the same thing to the UI and to the renderer. |
 
 ## `src/core/cutAnim.ts`
diff --git a/src/core/layerTree.ts b/src/core/layerTree.ts
index 9315d3b..2f5f7ab 100644
--- a/src/core/layerTree.ts
+++ b/src/core/layerTree.ts
@@ -13,10 +13,11 @@ export function flattenForCanvas<L extends LayerLike>(layers: L[]): L[] {
 
 // Cheap change signature for a layer's strokes (strokes are append/replace only here),
 // used to invalidate the layer canvas cache without stringifying the whole array.
-export function strokeSig(strokes: ReadonlyArray<StrokeLike> | null | undefined): string {
-    if (!strokes || !strokes.length) return '0';
-    const last = strokes[strokes.length - 1];
-    return strokes.length + '|' + (last.id ?? '') + '|' + (last.points ? last.points.length : 0) + '|' + (last.bitmapId ?? '') + '|' + (last.tool ?? '');
+export function strokeSig(strokes: ReadonlyArray<StrokeLike> | null | undefined, count?: number): string {
+    const n = Math.max(0, Math.min(count ?? strokes?.length ?? 0, strokes?.length ?? 0));
+    if (!strokes || !n) return '0';
+    const last = strokes[n - 1];
+    return n + '|' + (last.id ?? '') + '|' + (last.points ? last.points.length : 0) + '|' + (last.bitmapId ?? '') + '|' + (last.tool ?? '');
 }
 
 /**
@@ -32,13 +33,16 @@ export function strokeSig(strokes: ReadonlyArray<StrokeLike> | null | undefined)
  * `rev` is in it because edits that move coordinates without changing the stroke count or the
  * last stroke - a whole-layer move, say - are invisible to strokeSig.
  *
+ * `count` asks what the signature *would have been* when the layer held only its first `count`
+ * strokes. That is what lets `appendedAfter` recognise a layer that has only grown.
+ *
  * @param {{strokes?: any[], roughen?: number, rev?: number}} layer
  * @param {{roughPhase?: number, roughWave?: number, roughMinSize?: number} | null} [rough]
  *   the boiling options, when asking for a particular phase; omitted for the still frame
  * @returns {string}
  */
-export function layerSig(layer: LayerLike | null | undefined, rough: { roughPhase: number, roughWave: number, roughMinSize: number } | null = null): string {
-    const base = strokeSig(layer?.strokes) + '|r' + (layer?.roughen || 0) + '|v' + (layer?.rev || 0);
+export function layerSig(layer: LayerLike | null | undefined, rough: { roughPhase: number, roughWave: number, roughMinSize: number } | null = null, count?: number): string {
+    const base = strokeSig(layer?.strokes, count) + '|r' + (layer?.roughen || 0) + '|v' + (layer?.rev || 0);
     if (!rough || !layer?.roughen) return base;
     return base + `|b${rough.roughPhase}|w${rough.roughWave}|m${rough.roughMinSize}`;
 }
@@ -56,3 +60,36 @@ export function flattenLayersInUiOrder<L extends LayerLike>(layers: L[], parentI
     }
     return out;
 }
+
+/**
+ * How many of a layer's strokes a canvas baked under `stored` already holds - when the layer's
+ * strokes are that same list with more added on the end. `null` when they are not, and the
+ * canvas has to be redrawn from scratch.
+ *
+ * Committing a stroke redrew every stroke the layer held, so the Nth stroke of a drawing cost N
+ * times the first and a session's baking grew with the square of the strokes drawn. Measured on
+ * a 1920x1080 layer, drawing 40 brush strokes one at a time cost ~38s of baking where drawing
+ * only each new stroke costs ~1.9s.
+ *
+ * This leans on exactly the invariant the cache already needs, and no more: an edit that changes
+ * strokes without changing the count or the last stroke's identity must bump `rev` - see
+ * `offsetLayers`, which exists to do that. A prefix that changed under a cache that did not
+ * notice is already a stale cache today; this does not widen the assumption, it reuses it.
+ *
+ * @param {string|null|undefined} stored the signature the canvas was baked under
+ * @param {LayerLike|null|undefined} layer
+ * @param {{roughPhase: number, roughWave: number, roughMinSize: number}|null} [rough]
+ * @returns {number|null} the index to start drawing from
+ */
+export function appendedAfter(
+    stored: string | null | undefined,
+    layer: LayerLike | null | undefined,
+    rough: { roughPhase: number, roughWave: number, roughMinSize: number } | null = null,
+): number | null {
+    if (!stored) return null;
+    const have = layer?.strokes?.length ?? 0;
+    const was = Number(String(stored).split('|')[0]);
+    // Equal counts are the caller's cache hit, not an append; a shorter list is a removal.
+    if (!Number.isInteger(was) || was <= 0 || was >= have) return null;
+    return layerSig(layer, rough, was) === stored ? was : null;
+}
diff --git a/src/hooks/useLayerCache.ts b/src/hooks/useLayerCache.ts
index 5137b0d..8ba8f8e 100644
--- a/src/hooks/useLayerCache.ts
+++ b/src/hooks/useLayerCache.ts
@@ -2,7 +2,7 @@ import { useEffect, useRef, useState } from 'react';
 import { sizeCanvas, scratchCanvas } from '../canvas/scratch.ts';
 import { drawStrokesOnCtx } from '../canvas/strokes.ts';
 import { safeArray } from '../core/geometry.ts';
-import { layerKey, layerSig } from '../core/layerTree.ts';
+import { layerKey, layerSig, appendedAfter } from '../core/layerTree.ts';
 import { scanLayerBitmaps } from '../engine/pendingBitmaps.ts';
 import { cutsToCache } from '../engine/selectCuts.ts';
 import { layerKeysUsingBitmaps, keysWithPhases, prefetchWindow } from '../core/decodeBudget.ts';
@@ -246,8 +246,26 @@ export function useLayerCache({
         // which is why this does not go through scratchCanvas.
         const bake = (signature: string) => {
             const cnv = hit || document.createElement('canvas');
-            sizeCanvas(cnv, canvasW, canvasH);
-            drawStrokesOnCtx(cnv.getContext('2d')!, layer.strokes || [], true, store, rOpts);
+            // Read before sizing. Resizing a canvas clears it, and then nothing is already drawn
+            // however much the signature says is.
+            const stored = cnv.dataset.strokes;
+            const resized = sizeCanvas(cnv, canvasW, canvasH);
+            // Committing a stroke redrew every stroke the layer held, so the Nth stroke of a
+            // drawing cost N times the first and a session's baking grew with the square of the
+            // strokes drawn: on a 1920x1080 layer, 40 brush strokes drawn one at a time cost
+            // ~38s of baking where drawing only each new stroke costs ~1.9s.
+            //
+            // When the strokes are the ones already on this canvas plus more on the end, only
+            // the new ones need drawing - they paint on top, which is the order they would have
+            // been drawn in anyway. Everything else (a stroke edited or removed, the layer
+            // dragged, a merge) still bakes in full; appendedAfter says which this is.
+            //
+            // Boiling layers are left out: each phase keeps its own canvas, so an append would
+            // have to reach all of them.
+            const from = (resized || layer.roughen) ? null : appendedAfter(stored, layer, rOpts);
+            const target = cnv.getContext('2d')!;
+            if (from === null) drawStrokesOnCtx(target, layer.strokes || [], true, store, rOpts);
+            else drawStrokesOnCtx(target, (layer.strokes || []).slice(from), false, store, rOpts);
             cnv.dataset.strokes = signature;
             map.delete(slotKey); map.set(slotKey, cnv);   // re-insert = most recently used
             while (map.size > LAYER_CANVAS_LRU) map.delete(map.keys().next().value!);
diff --git a/test/canvas/pixels.test.js b/test/canvas/pixels.test.js
index 38b837c..5295303 100644
--- a/test/canvas/pixels.test.js
+++ b/test/canvas/pixels.test.js
@@ -130,6 +130,32 @@ test('an eraser still widens when the pen is pressed hard', () => {
     assert.ok(hard > even + 10, `a hard press did not widen the eraser: ${even} -> ${hard}`);
 });
 
+test('drawing only the new strokes gives the same pixels as redrawing the layer', () => {
+    // The claim the incremental bake rests on (appendedAfter): strokes paint in order, each on
+    // top of the last, so painting the new ones onto a canvas that already holds the old ones
+    // must land exactly where redrawing everything would. If this ever stops being true the
+    // symptom is a drawing that is quietly wrong, so it is checked on bytes, not on ink counts.
+    const all = [
+        stroke({ id: 1, tool: 'brush', color: '#cc3344' }),
+        stroke({ id: 2, tool: 'pencil', color: '#2244aa', points: [{ x: 30, y: 30, pressure: 0.5 }, { x: 170, y: 90, pressure: 0.5 }] }),
+        // An eraser in the appended part: destination-out has to compose against what is
+        // already on the canvas, which is the case most likely to come apart.
+        stroke({ id: 3, tool: 'eraser', size: 24, points: [{ x: 60, y: 10, pressure: 0.5 }, { x: 60, y: 110, pressure: 0.5 }] }),
+        stroke({ id: 4, tool: 'marker', color: '#117733', points: [{ x: 20, y: 90, pressure: 0.5 }, { x: 180, y: 70, pressure: 0.5 }] }),
+    ];
+    for (const split of [1, 2, 3]) {
+        const full = fresh(), incr = fresh();
+        drawStrokesOnCtx(full.ctx, all, true, new Map());
+        drawStrokesOnCtx(incr.ctx, all.slice(0, split), true, new Map());
+        drawStrokesOnCtx(incr.ctx, all.slice(split), false, new Map());
+        assert.deepEqual(
+            [...incr.ctx.getImageData(0, 0, W, H).data],
+            [...full.ctx.getImageData(0, 0, W, H).data],
+            `appending from ${split} did not match a full redraw`,
+        );
+    }
+});
+
 test('drawing is deterministic: the same stroke twice gives the same pixels', () => {
     const a = fresh(), b = fresh();
     drawStrokesOnCtx(a.ctx, [stroke()], false, new Map());
diff --git a/test/core/appendedAfter.test.js b/test/core/appendedAfter.test.js
new file mode 100644
index 0000000..2f79113
--- /dev/null
+++ b/test/core/appendedAfter.test.js
@@ -0,0 +1,94 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { layerSig, strokeSig, appendedAfter } from '../../src/core/layerTree.ts';
+
+// Committing a stroke redrew every stroke the layer held. The Nth stroke of a drawing cost N
+// times the first, so a session's baking grew with the square of the strokes drawn - measured on
+// a 1920x1080 layer, 40 brush strokes drawn one at a time cost ~38s of baking where drawing only
+// each new stroke costs ~1.9s. appendedAfter is what lets the cache draw only the new ones.
+//
+// Every interesting case here is a *refusal*. Saying "appended" when something else happened
+// leaves stale pixels on the layer with no symptom until someone notices their drawing is wrong,
+// so the tests that matter are the ones that make sure it says no.
+
+const stroke = (id, n = 4, over = {}) => ({
+    id, tool: 'brush', bitmapId: null,
+    points: Array.from({ length: n }, (_, i) => ({ x: i, y: i })),
+    ...over,
+});
+const layer = (strokes, over = {}) => ({ id: 1, strokes, rev: 0, roughen: 0, ...over });
+
+test('strokes added on the end are an append, starting where the canvas left off', () => {
+    const before = layer([stroke(1), stroke(2)]);
+    const after = layer([stroke(1), stroke(2), stroke(3), stroke(4)]);
+    assert.equal(appendedAfter(layerSig(before), after), 2);
+});
+
+test('an unchanged layer is not an append - that is the caller\'s cache hit', () => {
+    const l = layer([stroke(1), stroke(2)]);
+    assert.equal(appendedAfter(layerSig(l), l), null);
+});
+
+test('a removed stroke is not an append', () => {
+    const before = layer([stroke(1), stroke(2), stroke(3)]);
+    assert.equal(appendedAfter(layerSig(before), layer([stroke(1), stroke(2)])), null);
+});
+
+test('the last stroke growing points - the eraser still drawing - is not an append', () => {
+    // appendPoints extends the stroke in flight. Its pixels are already on the canvas and the
+    // new ones are not a separate stroke, so the layer has to be redrawn.
+    const before = layer([stroke(1), stroke(2, 4)]);
+    assert.equal(appendedAfter(layerSig(before), layer([stroke(1), stroke(2, 9)])), null);
+});
+
+test('a bumped rev refuses the append even though the strokes only grew', () => {
+    // This is the whole safety argument. offsetLayers moves every coordinate, which changes no
+    // count and no identity, and bumps rev precisely so the cache notices. If a move and a new
+    // stroke land together, the moved strokes must still be redrawn.
+    const before = layer([stroke(1), stroke(2)]);
+    const after = layer([stroke(1), stroke(2), stroke(3)], { rev: 1 });
+    assert.equal(appendedAfter(layerSig(before), after), null);
+});
+
+test('a layer that started empty is redrawn in full, not appended onto', () => {
+    assert.equal(appendedAfter(layerSig(layer([])), layer([stroke(1)])), null);
+});
+
+test('a missing or unparseable signature is refused', () => {
+    const l = layer([stroke(1), stroke(2)]);
+    for (const bad of [null, undefined, '', 'nonsense', '|1|2', 'NaN|x', '-1|x']) {
+        assert.equal(appendedAfter(bad, l), null, String(bad));
+    }
+});
+
+test('a signature claiming more strokes than the layer has is refused', () => {
+    // A canvas from a longer version of this layer. Drawing "the rest" would draw nothing and
+    // leave the extra strokes on screen.
+    const before = layer([stroke(1), stroke(2), stroke(3), stroke(4)]);
+    assert.equal(appendedAfter(layerSig(before), layer([stroke(1), stroke(2)])), null);
+});
+
+test('strokeSig with a count is the signature that list had when it was that long', () => {
+    // The property appendedAfter is built on: the prefix of a grown list signs as the original.
+    const first = [stroke(1), stroke(2)];
+    const grown = [...first, stroke(3)];
+    assert.equal(strokeSig(grown, 2), strokeSig(first));
+    assert.equal(layerSig(layer(grown), null, 2), layerSig(layer(first)));
+});
+
+test('a count outside the list is clamped rather than reading past the end', () => {
+    const l = [stroke(1), stroke(2)];
+    assert.equal(strokeSig(l, 99), strokeSig(l));
+    assert.equal(strokeSig(l, 0), '0');
+    assert.equal(strokeSig(l, -5), '0');
+});
+
+test('a boiling layer signs per phase, so a different phase is not an append', () => {
+    // Each phase keeps its own canvas and the bake path excludes boiling layers outright; this
+    // pins the signature half of that, so the exclusion cannot be quietly dropped.
+    const r1 = { roughPhase: 1, roughWave: 1, roughMinSize: 0 };
+    const r2 = { roughPhase: 2, roughWave: 1, roughMinSize: 0 };
+    const before = layer([stroke(1)], { roughen: 2 });
+    const after = layer([stroke(1), stroke(2)], { roughen: 2 });
+    assert.equal(appendedAfter(layerSig(before, r1), after, r2), null);
+});