SelectionAutoscroll should not happen in user-select:none.

third_party/WebKit/Source/core/input/MouseEventManager.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/MouseEventManager.h"
 
 #include "core/clipboard/DataObject.h"
 #include "core/clipboard/DataTransfer.h"
 #include "core/dom/Element.h"
 #include "core/dom/ElementTraversal.h"
@@ skipping 715 matching lines @@
       return WebInputEventResult::NotHandled;
 
     layoutObject = parent->layoutObject();
     if (!layoutObject || !layoutObject->isListBox())
       return WebInputEventResult::NotHandled;
   }
 
   m_mouseDownMayStartDrag = false;
 
   if (m_mouseDownMayStartAutoscroll &&
-      !m_scrollManager->middleClickAutoscrollInProgress()) {
+      !m_scrollManager->middleClickAutoscrollInProgress() &&
+      layoutObject->isSelectable()) {

[bokan, 2016/12/02 17:32:38]

Selection is created/added by the call below to handleMouseDraggedEvent right?
I'm wondering, would it work if we moved this scrolling to happen after that and
then just check whether or not we have a selection? I'm worried that this check
is too simplistic and there might be all sorts of other cases where we can't
select even though the layout object might be selectable. It would be better if
we can just rely on the selection system to worry about all the edge cases and
use its output to decide whether we should scroll.

[sunyunjia, 2016/12/04 22:46:43]

Done.

     if (AutoscrollController* controller =
             m_scrollManager->autoscrollController()) {
       controller->startAutoscrollForSelection(layoutObject);
       m_mouseDownMayStartAutoscroll = false;
     }
   }
 
   m_frame->eventHandler().selectionController().handleMouseDraggedEvent(
       event, m_mouseDownPos, m_dragStartPos, m_mousePressNode.get(),
       m_lastKnownMousePosition);
@@ skipping 244 matching lines @@
 
 void MouseEventManager::setClickCount(int clickCount) {
   m_clickCount = clickCount;
 }
 
 bool MouseEventManager::mouseDownMayStartDrag() {
   return m_mouseDownMayStartDrag;
 }
 
 }  // namespace blink