When clicking rapidly 2 different select popups, we could reach a state where

content/renderer/render_view_impl.cc

 // Copyright (c) 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/render_view_impl.h"
 
 #include <algorithm>
 #include <cmath>
 #include <string>
 #include <vector>
@@ skipping 1562 matching lines @@
   // TODO(jcivelli): Remove this deprecated method when its been removed from
   //                 the WebViewClient interface. It's been replaced by
   //                 createExternalPopupMenu.
   NOTREACHED();
   return NULL;
 }
 
 WebExternalPopupMenu* RenderViewImpl::createExternalPopupMenu(
     const WebPopupMenuInfo& popup_menu_info,
     WebExternalPopupMenuClient* popup_menu_client) {
-  DCHECK(!external_popup_menu_.get());
+  // An IPC message is sent to the browser to build and display the actual
+  // popup.  The user could have time to click a different select by the time
+  // the popup is shown.  In that case external_popup_menu_ is non NULL.
+  // By returning NULL in that case, we instruct WebKit to cancel that new
+  // popup.  So from the user perspective, only the first one will show, and
+  // will have to close the first one before another one can be shown.

[Ilya Sherman, 2012/05/14 23:26:04]

nit: Perhaps update this comment to mention that the |extenral_popup_menu_| can
also be non-NULL in case we haven't received the SelectPopupMenuItem message
yet?

+  if (external_popup_menu_.get())
+    return NULL;
   external_popup_menu_.reset(
       new ExternalPopupMenu(this, popup_menu_info, popup_menu_client));
   return external_popup_menu_.get();
 }
 
 RenderWidgetFullscreenPepper* RenderViewImpl::CreatePepperFullscreenContainer(
     webkit::ppapi::PluginInstance* plugin) {
   GURL active_url;
   if (webview() && webview()->mainFrame())
     active_url = GURL(webview()->mainFrame()->document().url());
@@ skipping 3727 matching lines @@
 bool RenderViewImpl::WebWidgetHandlesCompositorScheduling() const {
   return !!RenderThreadImpl::current()->compositor_thread();
 }
 
 void RenderViewImpl::OnJavaBridgeInit() {
   DCHECK(!java_bridge_dispatcher_);
 #if defined(ENABLE_JAVA_BRIDGE)
   java_bridge_dispatcher_ = new JavaBridgeDispatcher(this);
 #endif
 }