Add a comment explaining a different failure mode of the Pepper file chooser.

ppapi/c/dev/ppb_file_chooser_dev.h

 /* 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.
  */
 
-/* From dev/ppb_file_chooser_dev.idl modified Thu May 17 09:04:27 2012. */
+/* From dev/ppb_file_chooser_dev.idl modified Mon Jun  4 12:44:29 2012. */
 
 #ifndef PPAPI_C_DEV_PPB_FILE_CHOOSER_DEV_H_
 #define PPAPI_C_DEV_PPB_FILE_CHOOSER_DEV_H_
 
 #include "ppapi/c/pp_array_output.h"
 #include "ppapi/c/pp_bool.h"
 #include "ppapi/c/pp_completion_callback.h"
 #include "ppapi/c/pp_instance.h"
 #include "ppapi/c/pp_macros.h"
 #include "ppapi/c/pp_resource.h"
@@ skipping 73 matching lines @@
    */
   PP_Bool (*IsFileChooser)(PP_Resource resource);
   /**
    * This function displays a previously created file chooser resource as a
    * dialog box, prompting the user to choose a file or files. This function
    * must be called in response to a user gesture, such as a mouse click or
    * touch event. The callback is called with PP_OK on successful completion
    * with a file (or files) selected, PP_ERROR_USERCANCEL if the user selected
    * no file, or another error code from pp_errors.h on failure.
    *
+   * <b>Subtle note:</b> This function will only work when the tab containing
+   * the plugin is visible. Show() will fail if the tab is in the background.
+   * Since it's not normally possible to get input events while invisible, this
+   * is not normally an issue. But there is a race condition because events are
+   * processed asynchronously. If the user clicks and switches tabs very
+   * quickly, a plugin could believe the tab is visible while Chrome believes
+   * it is invisible and the Show() call will fail. This will not generally
+   * cause user confusion since the user will have switched tabs and will not
+   * want to see a file chooser from a different tab.
+   *
    * @param[in] chooser The file chooser resource.
    *
    * @param[in] output An output array which will receive PP_Resource(s)
    * identifying the <code>PPB_FileRef</code> objects that the user selected on
    * success.
    *
    * @param[in] callback A <code>CompletionCallback</code> to be called after
    * the user has closed the file chooser dialog.
    *
    * @return PP_OK_COMPLETIONPENDING if request to show the dialog was
@@ skipping 13 matching lines @@
   PP_Bool (*IsFileChooser)(PP_Resource resource);
   int32_t (*Show)(PP_Resource chooser, struct PP_CompletionCallback callback);
   PP_Resource (*GetNextChosenFile)(PP_Resource chooser);
 };
 /**
  * @}
  */
 
 #endif  /* PPAPI_C_DEV_PPB_FILE_CHOOSER_DEV_H_ */