Fix movementX/Y polyfill with capture events

packages/react-dom/src/events/SyntheticEvent.js

@@ -152,11 +152,22 @@ export const UIEventInterface: EventInterfaceType = {
   detail: 0,
 };
 
-let previousScreenX = 0;
-let previousScreenY = 0;
-// Use flags to signal movementX/Y has already been set
-let isMovementXSet = false;
-let isMovementYSet = false;
+let lastMovementX;
+let lastMovementY;
+let lastMouseEvent;
+
+function updateMouseMovementPolyfillState(event) {
+  if (event !== lastMouseEvent) {
+    if (lastMouseEvent && event.type === 'mousemove') {
+      lastMovementX = event.screenX - lastMouseEvent.screenX;
+      lastMovementY = event.screenY - lastMouseEvent.screenY;
+    } else {
+      lastMovementX = 0;
+      lastMovementY = 0;
+    }
+    lastMouseEvent = event;
+  }
+}
 
 /**
  * @interface MouseEvent
@@ -189,31 +200,17 @@ export const MouseEventInterface: EventInterfaceType = {
     if ('movementX' in event) {
       return event.movementX;
     }
-
-    const screenX = previousScreenX;
-    previousScreenX = event.screenX;
-
-    if (!isMovementXSet) {
-      isMovementXSet = true;
-      return 0;
-    }
-
-    return event.type === 'mousemove' ? event.screenX - screenX : 0;
+    updateMouseMovementPolyfillState(event);
+    return lastMovementX;
   },
   movementY: function(event) {
     if ('movementY' in event) {
       return event.movementY;
     }
-
-    const screenY = previousScreenY;
-    previousScreenY = event.screenY;
-
-    if (!isMovementYSet) {
-      isMovementYSet = true;
-      return 0;
-    }
-
-    return event.type === 'mousemove' ? event.screenY - screenY : 0;
+    // Don't need to call updateMouseMovementPolyfillState() here
+    // because it's guaranteed to have already run when movementX
+    // was copied.
+    return lastMovementY;
   },
 };
 

packages/react-dom/src/events/__tests__/SyntheticMouseEvent-test.js

@@ -16,6 +16,7 @@ describe('SyntheticMouseEvent', () => {
   let container;
 
   beforeEach(() => {
+    jest.resetModules();
     React = require('react');
     ReactDOM = require('react-dom');
 
@@ -77,4 +78,67 @@ describe('SyntheticMouseEvent', () => {
     expect(events[1]).toBe(6);
     expect(events[2]).toBe(0); // mousedown event should have movementX at 0
   });
+
+  it('should correctly calculate movementX/Y for capture phase', () => {
+    const events = [];
+    const onMouseMove = event => {
+      events.push(['move', false, event.movementX, event.movementY]);
+    };
+    const onMouseMoveCapture = event => {
+      events.push(['move', true, event.movementX, event.movementY]);
+    };
+    const onMouseDown = event => {
+      events.push(['down', false, event.movementX, event.movementY]);
+    };
+    const onMouseDownCapture = event => {
+      events.push(['down', true, event.movementX, event.movementY]);
+    };
+
+    const node = ReactDOM.render(
+      <div
+        onMouseMove={onMouseMove}
+        onMouseMoveCapture={onMouseMoveCapture}
+        onMouseDown={onMouseDown}
+        onMouseDownCapture={onMouseDownCapture}
+      />,
+      container,
+    );
+
+    let event = new MouseEvent('mousemove', {
+      relatedTarget: null,
+      bubbles: true,
+      screenX: 2,
+      screenY: 2,
+    });
+
+    node.dispatchEvent(event);
+
+    event = new MouseEvent('mousemove', {
+      relatedTarget: null,
+      bubbles: true,
+      screenX: 8,
+      screenY: 9,
+    });
+
+    node.dispatchEvent(event);
+
+    // Now trigger a mousedown event to see if movementX has changed back to 0
+    event = new MouseEvent('mousedown', {
+      relatedTarget: null,
+      bubbles: true,
+      screenX: 25,
+      screenY: 65,
+    });
+
+    node.dispatchEvent(event);
+
+    expect(events).toEqual([
+      ['move', true, 0, 0],
+      ['move', false, 0, 0],
+      ['move', true, 6, 7],
+      ['move', false, 6, 7],
+      ['down', true, 0, 0],
+      ['down', false, 0, 0],
+    ]);
+  });
 });