Disable scroll anchoring if the user engages in a drag motion

third_party/WebKit/Source/core/input/PointerEventManager.cpp

 // 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.
 
 #include "core/input/PointerEventManager.h"
 
 #include "core/dom/ElementTraversal.h"
 #include "core/dom/ExecutionContextTask.h"
 #include "core/dom/shadow/FlatTreeTraversal.h"
 #include "core/events/MouseEvent.h"
@@ skipping 135 matching lines @@
         && m_nodeUnderPointer.contains(pointerId)) {
         EventTarget* targetUnderPointer =
             m_nodeUnderPointer.get(pointerId).target;
         if (targetUnderPointer == target) {
             m_nodeUnderPointer.set(pointerId, EventTargetAttributes
                 (targetUnderPointer,
                 eventType == EventTypeNames::pointerover));
         }
     }
 
+    if (pointerEvent->type() == EventTypeNames::pointermove
+        && m_pointerEventFactory.isActiveButtonsState(pointerEvent->pointerId())) {
+        // If we see a pointermove while a button is down, presumably the user
+        // is engaging in drag motion.
+        // TODO(ymalik): This will need to be updated to account for pointermove
+        // events that occur within the slop region (see crbug.com/632067).
+        m_frame->view()->setUserMayHaveDragged(true);

[Navid Zolghadr, 2016/08/04 18:11:00]

Maybe for the events that cannot possibly cause scroll you can do this on
pointerdown?
This field (which we haven't added yet and it is unrelated to the change for the
slop region) could be useful in that case. I don't think we have a crbug for
that thought. Just something to keep in mind.

+    }
+
     if (!RuntimeEnabledFeatures::pointerEventEnabled())
         return WebInputEventResult::NotHandled;
     if (!checkForListener || target->hasEventListeners(eventType)) {
         UseCounter::count(m_frame->document(), UseCounter::PointerEventDispatch);
         if (eventType == EventTypeNames::pointerdown)
             UseCounter::count(m_frame->document(), UseCounter::PointerEventDispatchPointerDown);
 
         DispatchEventResult dispatchResult = target->dispatchEvent(pointerEvent);
         return EventHandler::toWebInputEventResult(dispatchResult);
     }
@@ skipping 712 matching lines @@
 {
     visitor->trace(m_frame);
     visitor->trace(m_nodeUnderPointer);
     visitor->trace(m_pointerCaptureTarget);
     visitor->trace(m_pendingPointerCaptureTarget);
     visitor->trace(m_touchEventManager);
 }
 
 
 } // namespace blink