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

chrome/browser/resources/pdf/gesture_detector.js

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 'use strict';
 
 /**
  * A class that listens for touch events and produces events when these
  * touches form gestures (e.g. pinching).
  */
 class GestureDetector {
   /**
    * Constructs a GestureDetector.
    * @param {!Element} element The element to monitor for touch gestures.
    */
   constructor(element) {
     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(
         'touchcancel', this.onTouch_.bind(this), { passive: true });
 
     this.pinchStartEvent_ = null;
     this.lastEvent_ = null;
 
     this.listeners_ = new Map([
       ['pinchstart', []],
@@ skipping 24 matching lines @@
     for (let l of listeners)
       l(pinchEvent);
   }
 
   /**
    * The callback for touchstart events on the element.
    * @private
    * @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_({
         type: 'pinchstart',
         center: GestureDetector.center_(event)
       });
     }
   }
 
   /**
@@ skipping 15 matching lines @@
         type: 'pinchend',
         startScaleRatio: startScaleRatio,
         center: center
       };
       this.pinchStartEvent_ = null;
       this.lastEvent_ = null;
       this.notify_(endEvent);
       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_);
     let center = GestureDetector.center_(event);
     this.notify_({
       type: 'pinchupdate',
       scaleRatio: scaleRatio,
       direction: scaleRatio > 1.0 ? 'in' : 'out',
       startScaleRatio: startScaleRatio,
       center: center
@@ skipping 39 matching lines @@
    */
   static center_(event) {
     let touch1 = event.touches[0];
     let touch2 = event.touches[1];
     return {
       x: (touch1.clientX + touch2.clientX) / 2,
       y: (touch1.clientY + touch2.clientY) / 2
     };
   }
 };