Bug 779358 - Fix mousemove event handled twice in content panning, r=cjones

dom/browser-element/BrowserElementScrolling.js

@@ -5,7 +5,7 @@
 const ContentPanning = {
   init: function cp_init() {
     ['mousedown', 'mouseup', 'mousemove'].forEach(function(type) {
-      addEventListener(type, ContentPanning, true);
+      addEventListener(type, ContentPanning, false);
     });
 
     addMessageListener("Viewport:Change", this._recvViewportChange.bind(this));
@@ -30,7 +30,7 @@ const ContentPanning = {
         let target = evt.target;
         let view = target.ownerDocument ? target.ownerDocument.defaultView
                                         : target;
-        view.removeEventListener('click', this, true, true);
+        view.removeEventListener('click', this, false, true);
         break;
     }
   },
@@ -83,7 +83,7 @@ const ContentPanning = {
       let target = this.target;
       let view = target.ownerDocument ? target.ownerDocument.defaultView
                                       : target;
-      view.addEventListener('click', this, true, true);
+      view.addEventListener('click', this, false, true);
     }
 
     if (this.panning)
@@ -107,6 +107,8 @@ const ContentPanning = {
       this.panning = true;
       this._resetActive();
     }
+    evt.stopPropagation();
+    evt.preventDefault();
   },