Re-add snap-back behavior (Chrome side)

content/renderer/webscrollbarbehavior_impl_gtkoraura.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 "third_party/WebKit/public/platform/WebPoint.h"
+#include "third_party/WebKit/public/platform/WebRect.h"
+
+namespace content {
+
+bool WebScrollbarBehaviorImpl::shouldCenterOnThumb(
+      blink::WebScrollbarBehavior::Button mouseButton,
+      bool shiftKeyPressed,
+      bool altKeyPressed) {
+#if defined(TOOLKIT_GTK) || (defined(OS_LINUX) && !defined(OS_CHROMEOS))
+  if (mouseButton == blink::WebScrollbarBehavior::ButtonMiddle)
+    return true;
+#endif
+  return (mouseButton == blink::WebScrollbarBehavior::ButtonLeft) &&
+      shiftKeyPressed;
+}
+
+bool WebScrollbarBehaviorImpl::shouldSnapBackToDragOrigin(
+    const blink::WebPoint& eventPoint,
+    const blink::WebRect& scrollbarRect,
+    bool isHorizontal) {
+#if defined(TOOLKIT_GTK)
+  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;
+
+  // Find the rect within which we shouldn't snap, by expanding the track rect
+  // in both dimensions.
+  gfx::Rect noSnapRect(scrollbarRect);
+  const int thickness = isHorizontal ? noSnapRect.height() : noSnapRect.width();
+  noSnapRect.Inset(
+      (isHorizontal ? kOffEndMultiplier : kOffSideMultiplier) * -thickness,
+      (isHorizontal ? kOffSideMultiplier : kOffEndMultiplier) * -thickness);
+
+  // We should snap iff the event is outside our calculated rect.
+  return !noSnapRect.Contains(eventPoint);
+}
+
+}  // namespace content