Selection-autoscroll should be triggered when the pointer is in the border belt.

third_party/WebKit/Source/core/page/AutoscrollController.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2012 Digia Plc. and/or its subsidiary(-ies)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
@@ skipping 216 matching lines @@
 }
 
 void AutoscrollController::animate(double)
 {
     if (!m_autoscrollLayoutObject || !m_autoscrollLayoutObject->frame()) {
         stopAutoscroll();
         return;
     }
 
     EventHandler& eventHandler = m_autoscrollLayoutObject->frame()->eventHandler();
+    IntSize offset = m_autoscrollLayoutObject->calculateAutoscrollDirection(eventHandler.lastKnownMousePosition());
+    IntPoint selectionPoint = eventHandler.lastKnownMousePosition() + offset;
     switch (m_autoscrollType) {
     case AutoscrollForDragAndDrop:
         if (WTF::monotonicallyIncreasingTime() - m_dragAndDropAutoscrollStartTime > autoscrollDelay)
             m_autoscrollLayoutObject->autoscroll(m_dragAndDropAutoscrollReferencePosition);
         break;
     case AutoscrollForSelection:
         if (!eventHandler.mousePressed()) {
             stopAutoscroll();
             return;
         }
         eventHandler.updateSelectionForMouseDrag();
-        m_autoscrollLayoutObject->autoscroll(eventHandler.lastKnownMousePosition());
+        m_autoscrollLayoutObject->autoscroll(selectionPoint);
         break;
     case NoAutoscroll:
         break;
     case AutoscrollForMiddleClickCanStop:
     case AutoscrollForMiddleClick:
         DCHECK(RuntimeEnabledFeatures::middleClickAutoscrollEnabled());
         if (!middleClickAutoscrollInProgress()) {
             stopAutoscroll();
             return;
         }
@@ skipping 41 matching lines @@
     } else if (east) {
         view->setCursor(eastPanningCursor());
     } else if (west) {
         view->setCursor(westPanningCursor());
     } else {
         view->setCursor(middlePanningCursor());
     }
 }
 
 } // namespace blink