website/playground/panels.js/CodeMirrorPanel/constructor
class CodeMirrorPanel:  constructor() {
    super();
    this._textareaRef = React.createRef();
    this._codeMirror = null;
    this._cached = "";
    this._overlay = null;
    this.handleChange = this.handleChange.bind(this);
    this.handleFocus = this.handleFocus.bind(this);
    this.handleSelectionChange = this.handleSelectionChange.bind(this);
  }