PDF pinch-zoom: preventDefault on 2 finger touchmoves, not touchstarts.

Currently, to prevent native pinch-zoom from interfering with how we
handle pinch-zoom in the PDF viewer, we preventDefault the 2 finger
touchstart that corresponds to the gesture pinch start. However, this
also prevents 2 finger tap gestures.

We now preventDefault the 2 finger touchmoves so that native pinch-zoom
is still prevented, while 2 finger tap gestures are not affected.

BUG=716123
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2847893005
Cr-Commit-Position: refs/heads/master@{#468986}

chrome/browser/resources/pdf/gesture_detector.js

@@ -17,9 +17,9 @@ class GestureDetector {
     this.element_ = element;
 
     this.element_.addEventListener(
-        'touchstart', this.onTouchStart_.bind(this), { passive: false });
+        'touchstart', this.onTouchStart_.bind(this), { passive: true });
     this.element_.addEventListener(
-        'touchmove', this.onTouch_.bind(this), { passive: true });
+        'touchmove', this.onTouch_.bind(this), { passive: false });
     this.element_.addEventListener(
         'touchend', this.onTouch_.bind(this), { passive: true });
     this.element_.addEventListener(
@@ -64,10 +64,7 @@ class GestureDetector {
    * @param {!TouchEvent} event Touch event on the element.
    */
   onTouchStart_(event) {
-    // We must preventDefault if there is a two finger touch. By doing so
-    // native pinch-zoom does not interfere with our way of handling the event.
     if (event.touches.length == 2) {
-      event.preventDefault();
       this.pinchStartEvent_ = event;
       this.lastEvent_ = event;
       this.notify_({
@@ -103,6 +100,10 @@ class GestureDetector {
       return;
     }
 
+    // We must preventDefault two finger touchmoves. By doing so native
+    // pinch-zoom does not interfere with our way of handling the event.
+    event.preventDefault();
+
     let scaleRatio = GestureDetector.pinchScaleRatio_(event, this.lastEvent_);
     let startScaleRatio = GestureDetector.pinchScaleRatio_(
         event, this.pinchStartEvent_);

chrome/test/data/pdf/gesture_detector_test.js

@@ -182,24 +182,34 @@ chrome.test.runTests(function() {
       let gestureDetector = new GestureDetector(stubElement);
       let pinchListener = new PinchListener(gestureDetector);
 
-      // Ensure that the touchstart listener is not passive, otherwise the
+      // Ensure that the touchmove listener is not passive, otherwise the
       // call to preventDefault will be ignored. Since listeners could default
       // to being passive, we must set the value explicitly
       // (see crbug.com/675730).
-      for (let l of stubElement.listeners.get('touchstart')) {
+      for (let l of stubElement.listeners.get('touchmove')) {
         let options = stubElement.listenerOptions.get(l);
         chrome.test.assertTrue(!!options &&
                                typeof(options.passive) == 'boolean');
         chrome.test.assertFalse(options.passive);
       }
 
+      // We should not preventDefault the touchstart for 2 fingers, since this
+      // could just be a 2 finger tap.
       let pinchStartEvent = new MockTouchEvent('touchstart', [
         {clientX: 0, clientY: 0},
         {clientX: 0, clientY: 2}
       ]);
       stubElement.sendEvent(pinchStartEvent);
       chrome.test.assertEq('pinchstart', pinchListener.lastEvent.type);
-      chrome.test.assertTrue(pinchStartEvent.defaultPrevented);
+      chrome.test.assertFalse(pinchStartEvent.defaultPrevented);
+
+      let pinchUpdateEvent = new MockTouchEvent('touchmove', [
+        {clientX: 0, clientY: 0},
+        {clientX: 0, clientY: 4}
+      ]);
+      stubElement.sendEvent(pinchUpdateEvent);
+      chrome.test.assertEq('pinchupdate', pinchListener.lastEvent.type);
+      chrome.test.assertTrue(pinchUpdateEvent.defaultPrevented);
 
       chrome.test.succeed();
     }