Keep the selection of the text field when changed by JS.

Source/core/input/EventHandler.cpp

 /*
  * 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
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 344 matching lines @@
     if (singleClick)
         focusDocumentView();
 
     Node* innerNode = event.innerNode();
 
     m_mousePressNode = innerNode;
     m_dragStartPos = event.event().position();
 
     bool swallowEvent = false;
     m_mousePressed = true;
-    selectionController().initializeSelectionState();
 
     if (event.event().clickCount() == 2)
         swallowEvent = selectionController().handleMousePressEventDoubleClick(event);
     else if (event.event().clickCount() >= 3)
         swallowEvent = selectionController().handleMousePressEventTripleClick(event);
     else
         swallowEvent = selectionController().handleMousePressEventSingleClick(event);
 
     m_mouseDownMayStartAutoscroll = selectionController().mouseDownMayStartSelect()
         || (m_mousePressNode && m_mousePressNode->layoutBox() && m_mousePressNode->layoutBox()->canBeProgramaticallyScrolled());
@@ skipping 567 matching lines @@
             m_resizeScrollableArea->setInResizeMode(true);
             m_offsetFromResizeCorner = LayoutSize(m_resizeScrollableArea->offsetFromResizeCorner(p));
             invalidateClick();
             return true;
         }
     }
 
     m_frame->selection().setCaretBlinkingSuspended(true);
 
     bool swallowEvent = !dispatchMouseEvent(EventTypeNames::mousedown, mev.innerNode(), m_clickCount, mouseEvent, true);
+    // m_selectionInitiationState is initialized after dispatching mousedown event in order not to keep the selection by DOM APIs

[yosin_UTC9, 2015/07/06 01:57:30]

nit: Could you try to keep comments in 80 chars? It makes some tools happy, e.g.
codereview.chromium.org, git log and so on.

[Miyoung Shin(g), 2015/07/13 03:32:57]

Done.

+    // Because we can't give the user the chance to handle the selection by user action like dragging if we keep the selection in case of mousedown.
+    // FireFox also has the same behavior and it's more compatible with other browsers.
+    selectionController().initializeSelectionState();
     HitTestResult hitTestResult = hitTestResultInFrame(m_frame, mouseEvent.position(), HitTestRequest::ReadOnly);
     swallowEvent = swallowEvent || handleMouseFocus(MouseEventWithHitTestResults(mouseEvent, hitTestResult));
     m_capturesDragging = !swallowEvent || mev.scrollbar();
 
     // If the hit testing originally determined the event was in a scrollbar, refetch the MouseEventWithHitTestResults
     // in case the scrollbar widget was destroyed when the mouse event was handled.
     if (mev.scrollbar()) {
         const bool wasLastScrollBar = mev.scrollbar() == m_lastScrollbarUnderMouse.get();
         HitTestRequest request(HitTestRequest::ReadOnly | HitTestRequest::Active);
         mev = m_frame->document()->prepareMouseEvent(request, documentPoint, mouseEvent);
@@ skipping 1051 matching lines @@
     IntPoint tappedPosition = gestureEvent.position();
 
     if (m_clickNode && m_clickNode->isTextNode())
         m_clickNode = ComposedTreeTraversal::parent(*m_clickNode);
 
     PlatformMouseEvent fakeMouseDown(gestureEvent.position(), gestureEvent.globalPosition(),
         LeftButton, PlatformEvent::MousePressed, gestureEvent.tapCount(),
         static_cast<PlatformEvent::Modifiers>(modifiers | PlatformEvent::LeftButtonDown),
         PlatformMouseEvent::FromTouch,  gestureEvent.timestamp());
     bool swallowMouseDownEvent = !dispatchMouseEvent(EventTypeNames::mousedown, currentHitTest.innerNode(), gestureEvent.tapCount(), fakeMouseDown, true);
+    selectionController().initializeSelectionState();
     if (!swallowMouseDownEvent)
         swallowMouseDownEvent = handleMouseFocus(MouseEventWithHitTestResults(fakeMouseDown, currentHitTest));
     if (!swallowMouseDownEvent)
         swallowMouseDownEvent = handleMousePressEvent(MouseEventWithHitTestResults(fakeMouseDown, currentHitTest));
 
     if (currentHitTest.innerNode()) {
         ASSERT(gestureEvent.type() == PlatformEvent::GestureTap);
         HitTestResult result = currentHitTest;
         result.setToShadowHostIfInUserAgentShadowRoot();
         m_frame->chromeClient().onMouseDown(result.innerNode());
@@ skipping 1835 matching lines @@
 unsigned EventHandler::accessKeyModifiers()
 {
 #if OS(MACOSX)
     return PlatformEvent::CtrlKey | PlatformEvent::AltKey;
 #else
     return PlatformEvent::AltKey;
 #endif
 }
 
 } // namespace blink