feat: implement cursor-centric zoom functionality in viewer

js/main.js

@@ -292,6 +292,7 @@ function wireEvents() {
     clearTimeout(previewDebounce); previewDebounce = setTimeout(updatePreview, 80);
   };
   scaleUSlider.addEventListener('input', () => applyScaleU(posToScale(parseFloat(scaleUSlider.value))));
+  scaleUSlider.addEventListener('dblclick', () => applyScaleU(posToScale(parseFloat(scaleUSlider.defaultValue))));
   scaleUVal.addEventListener('change', () => applyScaleU(parseFloat(scaleUVal.value)));
 
   // Scale V — when lock is on, mirror to U
@@ -304,6 +305,7 @@ function wireEvents() {
     clearTimeout(previewDebounce); previewDebounce = setTimeout(updatePreview, 80);
   };
   scaleVSlider.addEventListener('input', () => applyScaleV(posToScale(parseFloat(scaleVSlider.value))));
+  scaleVSlider.addEventListener('dblclick', () => applyScaleV(posToScale(parseFloat(scaleVSlider.defaultValue))));
   scaleVVal.addEventListener('change', () => applyScaleV(parseFloat(scaleVVal.value)));
 
   // Lock toggle
@@ -412,6 +414,11 @@ function wireEvents() {
     brushRadius = parseFloat(exclBrushRadiusSlider.value) / 2;
     exclBrushRadiusVal.value = parseFloat(exclBrushRadiusSlider.value);
   });
+  exclBrushRadiusSlider.addEventListener('dblclick', () => {
+    exclBrushRadiusSlider.value = exclBrushRadiusSlider.defaultValue;
+    brushRadius = parseFloat(exclBrushRadiusSlider.value) / 2;
+    exclBrushRadiusVal.value = parseFloat(exclBrushRadiusSlider.value);
+  });
   exclBrushRadiusVal.addEventListener('change', () => {
     let diam = Math.max(0.2, Math.min(100, parseFloat(exclBrushRadiusVal.value) || 10));
     brushRadius = diam / 2;
@@ -424,6 +431,12 @@ function wireEvents() {
     exclThresholdVal.value = bucketThreshold;
     _lastHoverTriIdx = -1; // invalidate hover so next mousemove re-computes
   });
+  exclThresholdSlider.addEventListener('dblclick', () => {
+    exclThresholdSlider.value = exclThresholdSlider.defaultValue;
+    bucketThreshold = parseFloat(exclThresholdSlider.value);
+    exclThresholdVal.value = bucketThreshold;
+    _lastHoverTriIdx = -1;
+  });
   exclThresholdVal.addEventListener('change', () => {
     bucketThreshold = Math.max(0, Math.min(180, parseFloat(exclThresholdVal.value) || 20));
     exclThresholdSlider.value = bucketThreshold;
@@ -907,6 +920,17 @@ function linkSlider(slider, valInput, onChangeFn, livePreview = true) {
       previewDebounce = setTimeout(updatePreview, 80);
     }
   });
+  // Double-click resets to default value
+  slider.addEventListener('dblclick', () => {
+    slider.value = slider.defaultValue;
+    const v = parseFloat(slider.value);
+    const display = onChangeFn(v);
+    if (isSpan) valInput.textContent = display; else valInput.value = display;
+    if (livePreview) {
+      clearTimeout(previewDebounce);
+      previewDebounce = setTimeout(updatePreview, 80);
+    }
+  });
   if (!isSpan) {
     valInput.addEventListener('change', () => {
       const raw = parseFloat(valInput.value);

js/viewer.js

@@ -183,6 +183,32 @@ export function initViewer(canvas) {
   controls.enableDamping = true;
   controls.dampingFactor = 0.08;
   controls.screenSpacePanning = true;
+  controls.enableZoom = false; // we handle zoom ourselves for cursor-centric behaviour
+
+  // Cursor-centric zoom: zoom toward the mouse pointer instead of screen centre
+  renderer.domElement.addEventListener('wheel', (e) => {
+    e.preventDefault();
+    const rect = renderer.domElement.getBoundingClientRect();
+    const ndcX =  ((e.clientX - rect.left) / rect.width)  * 2 - 1;
+    const ndcY = -((e.clientY - rect.top)  / rect.height) * 2 + 1;
+
+    // World position under cursor before zoom
+    const before = new THREE.Vector3(ndcX, ndcY, 0).unproject(camera);
+
+    // Apply zoom
+    const factor = e.deltaY > 0 ? 1 / 1.1 : 1.1;
+    camera.zoom = Math.max(0.05, Math.min(200, camera.zoom * factor));
+    camera.updateProjectionMatrix();
+
+    // World position under cursor after zoom
+    const after = new THREE.Vector3(ndcX, ndcY, 0).unproject(camera);
+
+    // Shift camera + target so the world point stays under the cursor
+    const delta = before.clone().sub(after);
+    camera.position.add(delta);
+    controls.target.add(delta);
+    controls.update();
+  }, { passive: false });
 
   // Resize observer
   const resizeObserver = new ResizeObserver(() => onResize());