Disallow display of multiple experimental.extension.popup(...) windows...

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "unprivileged": true,
     "types": [
       {
         "id": "MessageSender",
         "type": "object",
         "description": "An object containing information about the script context that sent a message or request.",
         "properties": {
@@ skipping 116 matching lines @@
         "type": "function",
         "description": "Returns an array of the JavaScript 'window' objects for each of the pages running inside the current extension.",
         "parameters": [
           {
             "type": "object",
             "name": "fetchProperties",
             "optional": true,
             "properties": {
               "type": {
                 "type": "string",
-                "enum": ["tab", "infobar", "notification"],
+                "enum": ["tab", "infobar", "notification", "popup"],
                 "optional": true,
-                "description": "The type of view to get. If omitted, returns all views (including background pages and tabs). Valid values: 'tab', 'infobar', 'notification'."
+                "description": "The type of view to get. If omitted, returns all views (including background pages and tabs). Valid values: 'tab', 'infobar', 'notification', 'popup'."
               },
               "windowId": {
                 "type": "integer",
                 "optional": true,
                 "description": "The window to restrict the search to. If omitted, returns all views."
               }
             }
           }
         ],
         "returns": {
@@ skipping 2625 matching lines @@
         "description": "Triggers a paste operation in the specified tab.",
         "parameters": [
           {"type": "integer", "name": "tabId", "minimum": 0},
           {"type": "function", "name": "callback", "optional": true, "parameters": []}
         ]
       }
     ],
     "events": []
   }
 ]