Prevent AutoscrollForSelection when selecting text in a fixed-position element.

third_party/WebKit/Source/core/layout/LayoutBox.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc.
  *               All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 1024 matching lines @@
   else if (windowAutoscrollPoint.y() > windowBox.maxY() - autoscrollBeltSize)
     windowAutoscrollPoint.move(0, autoscrollBeltSize);
 
   return windowAutoscrollPoint - pointInRootFrame;
 }
 
 LayoutBox* LayoutBox::findAutoscrollable(LayoutObject* layoutObject) {
   while (
       layoutObject &&
       !(layoutObject->isBox() && toLayoutBox(layoutObject)->canAutoscroll())) {
+    // Do not start autoscroll when the node is inside a fixed-position element.
+    if (layoutObject->isBox() && toLayoutBox(layoutObject)->hasLayer() &&
+        toLayoutBox(layoutObject)->layer()->scrollsWithViewport()) {
+      return nullptr;
+    }
+
     if (!layoutObject->parent() &&
         layoutObject->node() == layoutObject->document() &&
         layoutObject->document().localOwner())
       layoutObject = layoutObject->document().localOwner()->layoutObject();
     else
       layoutObject = layoutObject->parent();
   }
 
-  return layoutObject && layoutObject->isBox() ? toLayoutBox(layoutObject) : 0;
+  return layoutObject && layoutObject->isBox() ? toLayoutBox(layoutObject)
+                                               : nullptr;
 }
 
 static inline int adjustedScrollDelta(int beginningDelta) {
   // This implemention matches Firefox's.
   // http://mxr.mozilla.org/firefox/source/toolkit/content/widgets/browser.xml#856.
   const int speedReducer = 12;
 
   int adjustedDelta = beginningDelta / speedReducer;
   if (adjustedDelta > 1)
     adjustedDelta = static_cast<int>(adjustedDelta *
@@ skipping 4568 matching lines @@
   LayoutRect rect = frameRect();
 
   LayoutBlock* block = containingBlock();
   if (block)
     block->adjustChildDebugRect(rect);
 
   return rect;
 }
 
 }  // namespace blink