make zooming zoom toward the mouse cursor

static/viewport.js

@@ -14,6 +14,23 @@ export class Viewport {
         this.zoomLevel = Math.max(-16, Math.min(20, this.zoomLevel));
     }
 
+    // ndcX and ndcY are in -0.5 .. 0.5
+    //  -0.5 -> (x) left  or (y) top
+    //   0.5 -> (x) right or (y) bottom
+    // (0, 0) is the center of the screen
+    zoomIntoPoint(delta, ndcX, ndcY, windowSize) {
+        let beforePixelsX = (ndcX * windowSize.width) / this.zoom;
+        let beforePixelsY = (ndcY * windowSize.height) / this.zoom;
+
+        this.zoomIn(delta);
+
+        let afterPixelsX = (ndcX * windowSize.width) / this.zoom;
+        let afterPixelsY = (ndcY * windowSize.height) / this.zoom;
+
+        this.panX += beforePixelsX - afterPixelsX;
+        this.panY += beforePixelsY - afterPixelsY;
+    }
+
     getVisibleRect(windowSize) {
         let invZoom = 1 / this.zoom;
         let width = windowSize.width * invZoom;