Add transient message data for display updates to the message spec.

packages/services/src/kernel/messages.ts

@@ -141,6 +141,7 @@ namespace KernelMessage {
     content: {
       data: nbformat.IMimeBundle,
       metadata: nbformat.OutputMetadata;
+      transient?: {display_id?: string};
     };
   }
 
@@ -152,6 +153,26 @@ namespace KernelMessage {
     return msg.header.msg_type === 'display_data';
   }
 
+  /**
+   * An `'update_display_data'` message on the `'iopub'` channel.
+   *
+   * See [Update Display data](https://jupyter-client.readthedocs.io/en/latest/messaging.html#update-display-data).
+   */
+  export
+  interface IUpdateDisplayDataMsg extends IDisplayDataMsg {
+    content: IDisplayDataMsg['content'] & {
+      transient: {display_id: string};
+    };
+  }
+
+  /**
+   * Test whether a kernel message is an `'update_display_data'` message.
+   */
+  export
+  function isUpdateDisplayDataMsg(msg: IMessage): msg is IUpdateDisplayDataMsg {
+    return msg.header.msg_type === 'update_display_data';
+  }
+
   /**
    * An `'execute_input'` message on the `'iopub'` channel.
    *