Disable scrollbar snapback on desktop Linux (Reland)

content/renderer/webscrollbarbehavior_impl_aura.cc

 // Copyright 2014 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 "content/renderer/webscrollbarbehavior_impl_gtkoraura.h"
+#include "content/renderer/webscrollbarbehavior_impl_aura.h"
 
 #include "build/build_config.h"
 #include "third_party/WebKit/public/platform/WebPoint.h"
 #include "third_party/WebKit/public/platform/WebRect.h"
 
 namespace content {
 
 bool WebScrollbarBehaviorImpl::shouldCenterOnThumb(
       blink::WebPointerProperties::Button mouseButton,
       bool shiftKeyPressed,
       bool altKeyPressed) {
 #if (defined(OS_LINUX) && !defined(OS_CHROMEOS))
   if (mouseButton == blink::WebPointerProperties::Button::Middle)
     return true;
 #endif
   return (mouseButton == blink::WebPointerProperties::Button::Left) &&
       shiftKeyPressed;
 }
 
 bool WebScrollbarBehaviorImpl::shouldSnapBackToDragOrigin(
     const blink::WebPoint& eventPoint,
     const blink::WebRect& scrollbarRect,
     bool isHorizontal) {
+// Disable snapback on desktop Linux to better integrate with the desktop
+// behavior.  Typically, Linux apps do not implement scrollbar snapback (this is
+// true for at least GTK and QT apps).
+#if (defined(OS_LINUX) && !defined(OS_CHROMEOS))
+  return false;
+#endif
+
   // Constants used to figure the drag rect outside which we should snap the
   // scrollbar thumb back to its origin. These calculations are based on
   // observing the behavior of the MSVC8 main window scrollbar + some
   // guessing/extrapolation.
   static const int kOffEndMultiplier = 3;
   static const int kOffSideMultiplier = 8;
   static const int kDefaultWinScrollbarThickness = 17;
 
   // Find the rect within which we shouldn't snap, by expanding the track rect
   // in both dimensions.
   gfx::Rect noSnapRect(scrollbarRect);
   int thickness = isHorizontal ? noSnapRect.height() : noSnapRect.width();
   // Even if the platform's scrollbar is narrower than the default Windows one,
   // we still want to provide at least as much slop area, since a slightly
   // narrower scrollbar doesn't necessarily imply that users will drag
   // straighter.
   thickness = std::max(thickness, kDefaultWinScrollbarThickness);
   noSnapRect.Inset(
       (isHorizontal ? kOffEndMultiplier : kOffSideMultiplier) * -thickness,
       (isHorizontal ? kOffSideMultiplier : kOffEndMultiplier) * -thickness);
 
-  // On most platforms, we should snap iff the event is outside our calculated
-  // rect.  On Linux, however, we should not snap for events off the ends, but
-  // not the sides, of the rect.
-#if (defined(OS_LINUX) && !defined(OS_CHROMEOS))
-  return isHorizontal ?
-      (eventPoint.y < noSnapRect.y() || eventPoint.y >= noSnapRect.bottom()) :
-      (eventPoint.x < noSnapRect.x() || eventPoint.x >= noSnapRect.right());
-#else
   return !noSnapRect.Contains(eventPoint);
-#endif
 }
 
 }  // namespace content