Don't route mouse wheel events to transparent children (Legacy_RenderWidgetHostHWND).

ui/base/win/mouse_wheel_util.cc

 // Copyright (c) 2009 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 "ui/base/win/mouse_wheel_util.h"
 
 #include <windowsx.h>
 
 #include "base/auto_reset.h"
 #include "ui/base/view_prop.h"
@@ skipping 83 matching lines @@
       } else {
         // The wheel is scrolling over an unrelated window. Make sure that we
         // have marked that window as supporting mouse wheel rerouting.
         // Otherwise, we cannot send random WM_MOUSEWHEEL messages to arbitrary
         // windows. So just drop the message.
         if (!WindowSupportsRerouteMouseWheel(window_under_wheel))
           return true;
       }
     }
 
+    // If the child window is transparent, then it is not interested in
+    // receiving wheel events.
+    if (IsChild(window, window_under_wheel) &&
+        ::GetWindowLong(
+            window_under_wheel, GWL_EXSTYLE) & WS_EX_TRANSPARENT) {
+      return false;
+    }
+
     // window_under_wheel is a Chrome window.  If allowed, redirect.
     if (IsCompatibleWithMouseWheelRedirection(window_under_wheel)) {
       base::AutoReset<bool> auto_reset_recursion_break(&recursion_break, true);
       SendMessage(window_under_wheel, WM_MOUSEWHEEL, w_param, l_param);
       return true;
     }
     // If redirection is disallowed, try the parent.
     window_under_wheel = GetAncestor(window_under_wheel, GA_PARENT);
   }
   // If we traversed back to the starting point, we should process
   // this message normally; return false.
   return false;
 }
 
 }  // namespace ui