Add drag-to-reposition for aggressive crops

Non-original aspects (square, 4:5, 16:9) always centered the crop with
no way to choose what got cut off. Crop position is now a normalized
0-1 offset per axis instead of a fixed /2 center, and dragging directly
on the canvas -- only when the current aspect actually leaves slack to
pan within -- adjusts it. Pointer movement converts to rotated-image
pixels via the crop-width-to-display-width ratio, so it stays 1:1
regardless of the maxDim downscale or the canvas's own CSS scaling.
Resets to centered on aspect or rotation change, since old offsets
don't make sense against new dimensions. Verified with a real drag test:
a marker cropped out at the centered default becomes visible after
dragging, and the "original" aspect (no crop, no slack) correctly
leaves panning disabled.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app.js

@@ -5,6 +5,7 @@
   const editorCard = document.getElementById('editor-card');
   const canvas = document.getElementById('canvas');
   const ctx = canvas.getContext('2d');
+  const panHint = document.getElementById('pan-hint');
 
   const aspectRow = document.getElementById('aspect-row');
   const rotateLeftBtn = document.getElementById('rotate-left-btn');
@@ -52,6 +53,11 @@
   let originalImg = null;
   let exposureImg = null;
 
+  // Slack the current rotation+aspect leaves to pan within, in rotated-
+  // image pixel space — recomputed every rebuildSource(), read by the
+  // canvas drag handler to convert pointer movement into cropOffset.
+  let cropBounds = { rotatedW: 0, rotatedH: 0, cropW: 0, cropH: 0 };
+
   const PRESETS = {
     none:     { filter: 'none' },
     // "Pick a vibe" presets rather than raw white-balance sliders, per
@@ -74,6 +80,12 @@
   const params = {
     rotation: 0,
     cropAspect: 'original',
+    // Normalized 0-1 position of the crop window within whatever slack the
+    // chosen aspect leaves — 0.5/0.5 is centered (the old fixed behavior).
+    // Stored as a fraction rather than raw pixels so it survives rotation/
+    // aspect changes without needing to be re-derived.
+    cropOffsetX: 0.5,
+    cropOffsetY: 0.5,
     preset: 'none',
     grain: 0,
     grainSize: 40,
@@ -122,6 +134,8 @@
   function resetParams() {
     params.rotation = 0;
     params.cropAspect = 'original';
+    params.cropOffsetX = 0.5;
+    params.cropOffsetY = 0.5;
     params.preset = 'none';
     params.grain = 0;
     params.grainSize = 40;
@@ -201,12 +215,14 @@
       const currentAspect = rotatedW / rotatedH;
       if (currentAspect > targetAspect) {
         cropW = Math.round(rotatedH * targetAspect);
-        cropX = Math.round((rotatedW - cropW) / 2);
+        cropX = Math.round((rotatedW - cropW) * params.cropOffsetX);
       } else {
         cropH = Math.round(rotatedW / targetAspect);
-        cropY = Math.round((rotatedH - cropH) / 2);
+        cropY = Math.round((rotatedH - cropH) * params.cropOffsetY);
       }
     }
+    cropBounds = { rotatedW, rotatedH, cropW, cropH };
+    updatePanAffordance();
 
     // Cap so a raw phone photo doesn't choke canvas ops.
     const maxDim = 2400;
@@ -233,6 +249,70 @@
     channelCanvas.height = outH;
   }
 
+  function hasPanSlack() {
+    return cropBounds.rotatedW > cropBounds.cropW || cropBounds.rotatedH > cropBounds.cropH;
+  }
+
+  function updatePanAffordance() {
+    const pannable = hasPanSlack();
+    canvas.classList.toggle('pannable', pannable);
+    panHint.hidden = !pannable;
+  }
+
+  // Drag-to-reposition: only meaningful once a non-"original" aspect has
+  // actually cropped something away. Converts pointer movement in CSS
+  // pixels to rotated-image pixels via the crop width/display width
+  // ratio, so it stays 1:1 regardless of the maxDim downscale or the
+  // canvas's own CSS max-width/max-height scaling.
+  let dragState = null;
+  let panRaf = null;
+
+  canvas.addEventListener('pointerdown', (e) => {
+    if (!hasPanSlack()) return;
+    const { rotatedW, rotatedH, cropW, cropH } = cropBounds;
+    dragState = {
+      pointerId: e.pointerId,
+      startX: e.clientX,
+      startY: e.clientY,
+      cropX0: Math.round((rotatedW - cropW) * params.cropOffsetX),
+      cropY0: Math.round((rotatedH - cropH) * params.cropOffsetY),
+    };
+    canvas.setPointerCapture(e.pointerId);
+    canvas.classList.add('panning');
+  });
+
+  canvas.addEventListener('pointermove', (e) => {
+    if (!dragState || e.pointerId !== dragState.pointerId) return;
+    const rect = canvas.getBoundingClientRect();
+    const { rotatedW, rotatedH, cropW, cropH } = cropBounds;
+    const rotatedPxPerCssPxX = cropW / rect.width;
+    const rotatedPxPerCssPxY = cropH / rect.height;
+    const deltaX = (e.clientX - dragState.startX) * rotatedPxPerCssPxX;
+    const deltaY = (e.clientY - dragState.startY) * rotatedPxPerCssPxY;
+
+    const slackX = rotatedW - cropW;
+    const slackY = rotatedH - cropH;
+    const newCropX = Math.min(Math.max(dragState.cropX0 - deltaX, 0), slackX);
+    const newCropY = Math.min(Math.max(dragState.cropY0 - deltaY, 0), slackY);
+    params.cropOffsetX = slackX > 0 ? newCropX / slackX : 0.5;
+    params.cropOffsetY = slackY > 0 ? newCropY / slackY : 0.5;
+
+    if (panRaf) return;
+    panRaf = requestAnimationFrame(() => {
+      panRaf = null;
+      rebuildSource();
+      render();
+    });
+  });
+
+  function endPan(e) {
+    if (!dragState || (e && e.pointerId !== dragState.pointerId)) return;
+    canvas.classList.remove('panning');
+    dragState = null;
+  }
+  canvas.addEventListener('pointerup', endPan);
+  canvas.addEventListener('pointercancel', endPan);
+
   // Recomputes the full stack from `source` every time — never chains
   // operations onto already-processed pixels — so dragging any slider
   // back to 0 reproduces the source exactly.
@@ -531,6 +611,8 @@
   [...aspectRow.children].forEach(btn => {
     btn.addEventListener('click', () => {
       params.cropAspect = btn.dataset.aspect;
+      params.cropOffsetX = 0.5;
+      params.cropOffsetY = 0.5;
       [...aspectRow.children].forEach(b => b.classList.toggle('active', b === btn));
       rebuildSource();
       render();
@@ -539,11 +621,15 @@
 
   rotateLeftBtn.addEventListener('click', () => {
     params.rotation = (params.rotation + 270) % 360;
+    params.cropOffsetX = 0.5;
+    params.cropOffsetY = 0.5;
     rebuildSource();
     render();
   });
   rotateRightBtn.addEventListener('click', () => {
     params.rotation = (params.rotation + 90) % 360;
+    params.cropOffsetX = 0.5;
+    params.cropOffsetY = 0.5;
     rebuildSource();
     render();
   });