perf(canvas): rAF-coalesced render, dirty imageData, viewport-bounded zoom/pan

Mobile lag root cause: every pan/zoom tick ran putImageData(imageData, 0, 0),
a ~67MB copy into the offscreen. On a phone, that dominates the frame budget.

- CanvasRenderer: split render() into a deferred scheduler + doRender().
  requestAnimationFrame coalesces redundant calls within one frame. An
  imageDataDirty flag gates putImageData — only re-upload when pixels
  actually changed (setPixelRgba / loadCanvas). Pan+zoom renders are now
  pure compositor work.
- Zoom clamp: minZoom = min(viewportW/CW, viewportH/CH). Computed in
  App.svelte from a resize-tracked viewport, passed to CanvasRenderer
  (wheel + pinch use it) and CanvasControls (zoom-out button disables at
  bound). Replaces the hardcoded 0.25 that let phones see empty void.
- Pan clamp: clampPan() runs before every paint. When canvas >= viewport,
  canvas always covers the screen. When canvas < viewport (never happens
  at minZoom, but safe), canvas stays inside.

src/client/App.svelte

@@ -1,5 +1,5 @@
 <script>
-  import { MAX_BATCH_SIZE, REQUEST_COOLDOWN_SEC } from '../lib/constants.js';
+  import { MAX_BATCH_SIZE, REQUEST_COOLDOWN_SEC, CANVAS_WIDTH, CANVAS_HEIGHT } from '../lib/constants.js';
   import CanvasRenderer from './components/CanvasRenderer.svelte';
   import ColorPicker from './components/ColorPicker.svelte';
   import CanvasControls from './components/CanvasControls.svelte';
@@ -10,6 +10,25 @@
   let selectedColor = $state(0);
   let cursorPos = $state({ x: 0, y: 0 });
   let zoom = $state(1);
+
+  // Viewport-aware zoom bounds. minZoom = "canvas fits in viewport" — any
+  // further and the canvas would shrink past the screen, leaving empty
+  // void space. Recomputed on window resize.
+  let viewport = $state({
+    w: typeof window !== 'undefined' ? window.innerWidth : 1920,
+    h: typeof window !== 'undefined' ? window.innerHeight : 1080,
+  });
+  const MAX_ZOOM = 64;
+  const minZoom = $derived(Math.min(viewport.w / CANVAS_WIDTH, viewport.h / CANVAS_HEIGHT));
+  const clampZoom = (z) => Math.max(minZoom, Math.min(MAX_ZOOM, z));
+  $effect(() => {
+    if (typeof window === 'undefined') return;
+    const onResize = () => { viewport = { w: window.innerWidth, h: window.innerHeight }; };
+    window.addEventListener('resize', onResize);
+    return () => window.removeEventListener('resize', onResize);
+  });
+  // After viewport or zoom-bound changes, pull current zoom into range.
+  $effect(() => { const z = clampZoom(zoom); if (z !== zoom) zoom = z; });
   let mode = $state('paint');
   let submitting = $state(false);
   let bufferState = $state({ canUndo: false, canRedo: false, pixelCount: 0 });
@@ -150,11 +169,11 @@
         return;
       case 'q': case 'Q':
         e.preventDefault();
-        zoom = Math.min(zoom * 2, 64);
+        zoom = clampZoom(zoom * 2);
         return;
       case 'e': case 'E':
         e.preventDefault();
-        zoom = Math.max(zoom / 2, 0.25);
+        zoom = clampZoom(zoom / 2);
         return;
       case 'w': case 'W':
         e.preventDefault(); canvasRenderer?.panBy(0, PAN_STEP); return;
@@ -240,20 +259,22 @@
     bind:this={canvasRenderer}
     {selectedColor}
     {zoom}
+    {minZoom}
     {mode}
     pickActive={!!activePick}
     onPick={handleCanvasPick}
     onEyedrop={handleEyedrop}
-    onZoomChange={(z) => zoom = z}
+    onZoomChange={(z) => zoom = clampZoom(z)}
     onCursorMove={(pos) => cursorPos = pos}
     onBufferChange={(s) => bufferState = s}
     onBufferFull={handleBufferFull}
   />
   <CanvasControls
     {zoom}
-    onZoomIn={() => zoom = Math.min(zoom * 2, 64)}
+    {minZoom}
-    onZoomOut={() => zoom = Math.max(zoom / 2, 0.25)}
+    onZoomIn={() => zoom = clampZoom(zoom * 2)}
-    onResetZoom={() => zoom = 1}
+    onZoomOut={() => zoom = clampZoom(zoom / 2)}
+    onResetZoom={() => zoom = clampZoom(1)}
     onGoto={(x, y) => canvasRenderer?.gotoPoint(x, y)}
     {cursorPos}
     {wsState}

src/client/components/CanvasControls.svelte

@@ -1,9 +1,12 @@
 <script>
   import { CANVAS_WIDTH, CANVAS_HEIGHT } from '../../lib/constants.js';
 
-  let { zoom, onZoomIn, onZoomOut, onResetZoom, onGoto, cursorPos,
+  let { zoom, minZoom = 0.25, onZoomIn, onZoomOut, onResetZoom, onGoto, cursorPos,
         wsState = 'connecting', onHelp } = $props();
 
+  const canZoomOut = $derived(zoom > minZoom + 1e-6);
+  const canZoomIn = $derived(zoom < 64 - 1e-6);
+
   const wsLabel = $derived({
     open: 'Live',
     connecting: 'Connecting…',
@@ -34,9 +37,10 @@
 <div class="controls">
   <span class="ws {wsState}" title={wsLabel} aria-label="Connection: {wsLabel}"></span>
   <div class="zoom">
-    <button onclick={onZoomOut} title="Zoom out (E)">−</button>
+    <button onclick={onZoomOut} disabled={!canZoomOut}
+      title={canZoomOut ? 'Zoom out (E)' : 'At min zoom — canvas fits the viewport'}>−</button>
     <span class="level">{zoomLabel}</span>
-    <button onclick={onZoomIn} title="Zoom in (Q)">+</button>
+    <button onclick={onZoomIn} disabled={!canZoomIn} title="Zoom in (Q)">+</button>
     <button onclick={onResetZoom} title="Reset zoom">⟲</button>
   </div>
   <div class="coords">({cursorPos.x}, {cursorPos.y})</div>
@@ -90,9 +94,13 @@
     justify-content: center;
   }
 
-  .zoom button:hover {
+  .zoom button:hover:not(:disabled) {
     background: #555;
   }
+  .zoom button:disabled {
+    opacity: 0.35;
+    cursor: default;
+  }
 
   .level {
     min-width: 40px;

src/client/components/CanvasRenderer.svelte

@@ -6,7 +6,7 @@
   import { paletteToRgba } from '../../lib/image-to-palette.js';
 
   let { selectedColor, zoom, onZoomChange, onCursorMove, mode, onBufferChange, onBufferFull,
-        pickActive = false, onPick, onEyedrop } = $props();
+        pickActive = false, onPick, onEyedrop, minZoom = 0.25 } = $props();
 
   let canvasEl;
   let imageData = null;
@@ -36,15 +36,55 @@
   let touchStartTime = 0;
   let touchMoved = false;
 
+  // --- Render coalescing ---
+  // Any state change calls render(). Multiple calls within a frame collapse
+  // to one actual paint via requestAnimationFrame. imageDataDirty gates the
+  // expensive putImageData (16 MB copy): only re-upload when pixels changed.
+  let rafPending = false;
+  let imageDataDirty = true;
+  let pendingEffZoom = null;
+
   function render(effZoom = zoom) {
+    pendingEffZoom = effZoom;
+    if (rafPending) return;
+    rafPending = true;
+    requestAnimationFrame(() => {
+      rafPending = false;
+      const z = pendingEffZoom;
+      pendingEffZoom = null;
+      doRender(z);
+    });
+  }
+
+  function clampPan(effZoom = zoom) {
+    const vw = window.innerWidth;
+    const vh = window.innerHeight;
+    const dispW = CANVAS_WIDTH * effZoom;
+    const dispH = CANVAS_HEIGHT * effZoom;
+    // If canvas >= viewport on an axis, pan range is [vw - dispW, 0] — canvas
+    // always covers viewport. If canvas < viewport, range is [0, vw - dispW] —
+    // canvas always stays inside (centered at midpoint).
+    const minX = Math.min(0, vw - dispW);
+    const maxX = Math.max(0, vw - dispW);
+    const minY = Math.min(0, vh - dispH);
+    const maxY = Math.max(0, vh - dispH);
+    pan.x = Math.max(minX, Math.min(maxX, pan.x));
+    pan.y = Math.max(minY, Math.min(maxY, pan.y));
+  }
+
+  function doRender(effZoom) {
     if (!canvasEl || !imageData) return;
+    clampPan(effZoom);
     const dpr = window.devicePixelRatio || 1;
     const ctx = canvasEl.getContext('2d');
     ctx.imageSmoothingEnabled = false;
     ctx.setTransform(1, 0, 0, 1, 0, 0); // reset before clear
     ctx.fillStyle = '#1a1a1a';
     ctx.fillRect(0, 0, canvasEl.width, canvasEl.height);
+    if (imageDataDirty) {
       offCtx.putImageData(imageData, 0, 0);
+      imageDataDirty = false;
+    }
     ctx.setTransform(dpr, 0, 0, dpr, 0, 0); // CSS pixels → device pixels
     ctx.translate(pan.x, pan.y);
     ctx.scale(effZoom, effZoom);
@@ -71,6 +111,7 @@
     imageData.data[off + 1] = rgba[1];
     imageData.data[off + 2] = rgba[2];
     imageData.data[off + 3] = rgba[3];
+    imageDataDirty = true;
   }
 
   function notifyBuffer() {
@@ -302,7 +343,7 @@
   function handleWheel(e) {
     e.preventDefault();
     const factor = e.deltaY < 0 ? 2 : 0.5;
-    const newZoom = Math.max(0.25, Math.min(64, zoom * factor));
+    const newZoom = Math.max(minZoom, Math.min(64, zoom * factor));
     pan.x = e.clientX - (e.clientX - pan.x) * (newZoom / zoom);
     pan.y = e.clientY - (e.clientY - pan.y) * (newZoom / zoom);
     if (newZoom !== zoom) onZoomChange(newZoom);
@@ -365,7 +406,7 @@
       const dist = getTouchDist(e.touches);
       const center = getTouchCenter(e.touches);
       const scale = dist / lastTouchDist;
-      const newZoom = Math.max(0.25, Math.min(64, zoom * scale));
+      const newZoom = Math.max(minZoom, Math.min(64, zoom * scale));
       pan.x = center.x - (center.x - pan.x) * (newZoom / zoom);
       pan.y = center.y - (center.y - pan.y) * (newZoom / zoom);
       pan.x += center.x - lastMouse.x;
@@ -418,6 +459,7 @@
       committedColors = new Uint8Array(indices); // replace pre-allocated zero array
       const rgba = indicesToRgba(indices);
       imageData = new ImageData(rgba, CANVAS_WIDTH, CANVAS_HEIGHT);
+      imageDataDirty = true;
       render();
     } catch (err) {
       console.error('Failed to load canvas:', err);