contentChanged listener not firing

I’m developing an extension that uses a Jupyter notebook to teach students the syntax of a new language.

I need a listener to tell me when a cell’s contents change.
However, the contentChanged listener never seems to trigger.
Am I doing something wrong with how I set this up?

const extension: JupyterFrontEndPlugin<void> = {
  id: 'cell-output-logger',
  autoStart: true,
  requires: [INotebookTracker],
  activate: (app: JupyterFrontEnd, notebooks: INotebookTracker) => {
    console.log("demo extension loaded");
    notebooks.widgetAdded.connect((sender, notebookPanel) => {
      console.log('widgetAdded listener triggered. Note, I only expect this once per notebook!!!');
      if (!notebooks || !notebooks.currentWidget) { console.log('Notebooks misssing, or lack currentWidget!'); return; }
      const cell = notebookPanel.model?.cells.get(0);
      if (!cell) { console.log('missing cell'); return; }
      console.log('cell found, adding the listener')
      cell.contentChanged.connect((cell, _) => {
        console.log('This cell is listened to');
        console.log('cell', cell);