Flash players weren't receiving mouse wheel events.

third_party/WebKit/Source/web/WebPluginContainerImpl.cpp

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2014 Opera Software ASA. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 530 matching lines @@
     if (nodes.size() != 1)
         return false;
     return nodes.first().get() == m_element;
 }
 
 void WebPluginContainerImpl::requestTouchEventType(TouchEventRequestType requestType)
 {
     if (m_touchEventRequestType == requestType || !m_element)
         return;
 
-    if (m_element->document().frameHost()) {
-        EventHandlerRegistry& registry = m_element->document().frameHost()->eventHandlerRegistry();
+    if (FrameHost* frameHost = m_element->document().frameHost()) {
+        EventHandlerRegistry& registry = frameHost->eventHandlerRegistry();
         if (requestType != TouchEventRequestTypeNone && m_touchEventRequestType == TouchEventRequestTypeNone)
             registry.didAddEventHandler(*m_element, EventHandlerRegistry::TouchEventBlocking);
         else if (requestType == TouchEventRequestTypeNone && m_touchEventRequestType != TouchEventRequestTypeNone)
             registry.didRemoveEventHandler(*m_element, EventHandlerRegistry::TouchEventBlocking);
     }
     m_touchEventRequestType = requestType;
 }
 
 void WebPluginContainerImpl::setWantsWheelEvents(bool wantsWheelEvents)
 {
     if (m_wantsWheelEvents == wantsWheelEvents)
         return;
+    if (FrameHost* frameHost = m_element->document().frameHost()) {
+        EventHandlerRegistry& registry = frameHost->eventHandlerRegistry();
+        if (wantsWheelEvents)
+            registry.didAddEventHandler(*m_element, EventHandlerRegistry::WheelEventBlocking);
+        else
+            registry.didRemoveEventHandler(*m_element, EventHandlerRegistry::WheelEventBlocking);
+    }
+
     m_wantsWheelEvents = wantsWheelEvents;
     if (Page* page = m_element->document().page()) {
         if (ScrollingCoordinator* scrollingCoordinator = page->scrollingCoordinator()) {
             if (parent() && parent()->isFrameView())
                 scrollingCoordinator->notifyGeometryChanged();
         }
     }
 }
 
 WebPoint WebPluginContainerImpl::rootFrameToLocalPoint(const WebPoint& pointInRootFrame)
@@ skipping 138 matching lines @@
 #else
     dispose();
 #endif
 }
 
 void WebPluginContainerImpl::dispose()
 {
     m_isDisposed = true;
 
     requestTouchEventType(TouchEventRequestTypeNone);
+    setWantsWheelEvents(false);
 
     if (m_webPlugin) {
         RELEASE_ASSERT(!m_webPlugin->container() || m_webPlugin->container() == this);
         m_webPlugin->destroy();
         m_webPlugin = nullptr;
     }
 
     if (m_webLayer) {
         GraphicsLayer::unregisterContentsLayer(m_webLayer);
         m_webLayer = nullptr;
@@ skipping 244 matching lines @@
         // frame view.
         computeClipRectsForPlugin(m_element, windowRect, clipRect, unobscuredRect);
     }
     getPluginOcclusions(m_element, this->parent(), frameRect(), cutOutRects);
     // Convert to the plugin position.
     for (size_t i = 0; i < cutOutRects.size(); i++)
         cutOutRects[i].move(-frameRect().x(), -frameRect().y());
 }
 
 } // namespace blink