Applying patch from react-grid-layout#237

lib/Resizable.js

@@ -24,6 +24,9 @@ export default class Resizable extends React.Component<Props, void> {
   handleRefs: {[key: ResizeHandleAxis]: ReactRef<HTMLElement>} = {};
   lastHandleRect: ?ClientRect = null;
   slack: ?[number, number] = null;
+  // We add these two fields to remember the state at start of resizing
+  dragStart = null;
+  sizeStart = null;
 
   componentWillUnmount() {
     this.resetData();
@@ -87,48 +90,30 @@ export default class Resizable extends React.Component<Props, void> {
    * @return {Function}           Handler function.
    */
   resizeHandler(handlerName: 'onResize' | 'onResizeStart' | 'onResizeStop', axis: ResizeHandleAxis): Function {
-    return (e: SyntheticEvent<>, {node, deltaX, deltaY}: DragCallbackData) => {
+    // Using `lastX` and `lastY` rather than `deltaX` and `deltaY` (see longer comment below)
+    return (e: SyntheticEvent<>, {node, lastX, lastY}: DragCallbackData) => {
       // Reset data in case it was left over somehow (should not be possible)
-      if (handlerName === 'onResizeStart') this.resetData();
+      if (handlerName === 'onResizeStart') {
+        this.resetData();
+        // Remembering state at start of resizing
+        this.dragStart = { x: lastX, y: lastY };
+        this.sizeStart = { x: this.props.width, y: this.props.height };
+      }
 
       // Axis restrictions
       const canDragX = (this.props.axis === 'both' || this.props.axis === 'x') && axis !== 'n' && axis !== 's';
       const canDragY = (this.props.axis === 'both' || this.props.axis === 'y') && axis !== 'e' && axis !== 'w';
       // No dragging possible.
       if (!canDragX && !canDragY) return;
 
-      // Decompose axis for later use
-      const axisV = axis[0];
-      const axisH = axis[axis.length - 1]; // intentionally not axis[1], so that this catches axis === 'w' for example
-
-      // Track the element being dragged to account for changes in position.
-      // If a handle's position is changed between callbacks, we need to factor this in to the next callback.
-      // Failure to do so will cause the element to "skip" when resized upwards or leftwards.
-      const handleRect = node.getBoundingClientRect();
-      if (this.lastHandleRect != null) {
-        // If the handle has repositioned on either axis since last render,
-        // we need to increase our callback values by this much.
-        // Only checking 'n', 'w' since resizing by 's', 'w' won't affect the overall position on page,
-        if (axisH === 'w') {
-          const deltaLeftSinceLast = handleRect.left - this.lastHandleRect.left;
-          deltaX += deltaLeftSinceLast;
-        }
-        if (axisV === 'n') {
-          const deltaTopSinceLast = handleRect.top - this.lastHandleRect.top;
-          deltaY += deltaTopSinceLast;
-        }
-      }
-      // Storage of last rect so we know how much it has really moved.
-      this.lastHandleRect = handleRect;
-
-      // Reverse delta if using top or left drag handles.
-      if (axisH === 'w') deltaX = -deltaX;
-      if (axisV === 'n') deltaY = -deltaY;
-
-      // Update w/h by the deltas. Also factor in transformScale.
-      let width = this.props.width + (canDragX ? deltaX / this.props.transformScale : 0);
-      let height = this.props.height + (canDragY ? deltaY / this.props.transformScale : 0);
-
+      // In order to fix an issue where resizing does not correctly follow the mouse cursor, we use the `lastX` and
+      // `lastY` properties of the mouse event rather than the `deltaX` and `deltaY` properties.
+      //
+      // This approach is more robust because `lastX` and `lastY` are absolute values and will remain true even if some
+      // events get dropped, whereas calculating the resulting width with `deltaX` and `deltaY` in a cumulative way will
+      // accumulate errors if some events are skipped.
+      let width = this.sizeStart.x + (canDragX ? lastX - (this.dragStart.x ?? 0) : 0);
+      let height = this.sizeStart.y + (canDragY ? lastY - (this.dragStart.y ?? 0) : 0);
       // Run user-provided constraints.
       [width, height] = this.runConstraints(width, height);