Extension Tab Title is Blank after Closing+Reopen

I am working on a simple Jupyter Lab extension. I adapted the extension following this template:

activate: (app: JupyterFrontEnd, palette: ICommandPalette) => {
  console.log('JupyterLab extension jupyterlab_apod is activated!');

  // Create a blank content widget inside of a MainAreaWidget
  const content = new Widget();
  const widget = new MainAreaWidget({content});
  widget.id = 'apod-jupyterlab';
  widget.title.label = 'Astronomy Picture';
  widget.title.closable = true;

  // Add an application command
  const command: string = 'apod:open';
  app.commands.addCommand(command, {
    label: 'Random Astronomy Picture',
    execute: () => {
      if (!widget.isAttached) {
        // Attach the widget to the main work area if it's not there
        app.shell.add(widget, 'main');
      // Activate the widget

  // Add the command to the palette.
  palette.addItem({command, category: 'Tutorial'});

When I open the extension the first time, widget.title.label shows correctly. If I close and reopen, the title is blank. Does anyone know why this is happening, based on the code above?


It looks like you are creating a single global widget. When it is closed in the UX, that widget is disposed by default. Then when you open again, you are just attaching a disposed widget (which would have cleared the title and disposed its other resources).

Depending on how you want to do things, you can create a new widget each time the command is run, or you can intercept the close request to not dispose the widget, but to just remove it from the dock panel. I think creating a new widget each time is the more common pattern.

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