Ensure the scroll slop area is at least as large as it is by default on Windows.

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 {
@@ skipping 13 matching lines @@
 bool WebScrollbarBehaviorImpl::shouldSnapBackToDragOrigin(
     const blink::WebPoint& eventPoint,
     const blink::WebRect& scrollbarRect,
     bool isHorizontal) {
   // 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);
-  const int thickness = isHorizontal ? noSnapRect.height() : noSnapRect.width();
+  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