Browser plugin should never handle MouseWheels.

content/renderer/browser_plugin/browser_plugin.cc

 // Copyright 2012 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/browser_plugin/browser_plugin.h"
 
 #include <stddef.h>
 #include <utility>
 
 #include "base/command_line.h"
@@ skipping 430 matching lines @@
 }
 
 blink::WebInputEventResult BrowserPlugin::handleInputEvent(
     const blink::WebInputEvent& event,
     blink::WebCursorInfo& cursor_info) {
   if (guest_crashed_ || !attached())
     return blink::WebInputEventResult::NotHandled;
 
   DCHECK(!blink::WebInputEvent::isTouchEventType(event.type()));
 
-  if (event.type() == blink::WebInputEvent::MouseWheel) {
-    auto wheel_event = static_cast<const blink::WebMouseWheelEvent&>(event);
-    if (wheel_event.resendingPluginId == browser_plugin_instance_id_)
-      return blink::WebInputEventResult::NotHandled;
-  }
+  // With direct event routing turned on, BrowserPlugin should almost never
+  // see wheel events any more. The two exceptions are (1) scroll bubbling, and
+  // (2) synthetic mouse wheels generated by touchpad GesturePinch events on
+  // Mac, which always go to the mainframe and thus may hit BrowserPlugin if
+  // it's in a top-level embedder. In both cases we should indicate the event
+  // as not handled (for GesturePinch on Mac, indicating the event has been
+  // handled leads to touchpad pinch not working).
+  if (event.type() == blink::WebInputEvent::MouseWheel)
+    return blink::WebInputEventResult::NotHandled;
 
   if (blink::WebInputEvent::isGestureEventType(event.type())) {
     auto gesture_event = static_cast<const blink::WebGestureEvent&>(event);
     DCHECK(blink::WebInputEvent::GestureTapDown == event.type() ||
            gesture_event.resendingPluginId == browser_plugin_instance_id_);
 
     // We shouldn't be forwarding GestureEvents to the Guest anymore. Indicate
     // we handled this only if it's a non-resent event.
     return gesture_event.resendingPluginId == browser_plugin_instance_id_
                ? blink::WebInputEventResult::NotHandled
@@ skipping 155 matching lines @@
 
 bool BrowserPlugin::HandleMouseLockedInputEvent(
     const blink::WebMouseEvent& event) {
   BrowserPluginManager::Get()->Send(
       new BrowserPluginHostMsg_HandleInputEvent(browser_plugin_instance_id_,
                                                 &event));
   return true;
 }
 
 }  // namespace content