Selecting with shift+drag results in unexpected drag-n-drop

third_party/WebKit/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 311 matching lines @@
 
     m_frame->document()->updateStyleAndLayoutIgnorePendingStylesheets();
 
     if (FrameView* frameView = m_frame->view()) {
         if (frameView->isPointInScrollbarCorner(event.event().position()))
             return WebInputEventResult::NotHandled;
     }
 
     bool singleClick = event.event().clickCount() <= 1;
 
-    m_mouseDownMayStartDrag = singleClick && !isLinkSelection(event);
+    m_mouseDownMayStartDrag = singleClick && !isLinkSelection(event) && !isExtendingSelection(event);
 
     selectionController().handleMousePressEvent(event);
 
     m_mouseDown = event.event();
 
     if (m_frame->document()->isSVGDocument() && m_frame->document()->accessSVGExtensions().zoomAndPanEnabled()) {
         if (event.event().shiftKey() && singleClick) {
             m_svgPan = true;
             m_frame->document()->accessSVGExtensions().startPan(m_frame->view()->rootFrameToContents(event.event().position()));
             return WebInputEventResult::HandledSystem;
@@ skipping 2507 matching lines @@
 
 FrameHost* EventHandler::frameHost() const
 {
     if (!m_frame->page())
         return nullptr;
 
     return &m_frame->page()->frameHost();
 }
 
 } // namespace blink