website/playground/panels.js/CodeMirrorPanel/updateOverlay
class CodeMirrorPanel:  updateOverlay() {
    if (!this.props.readOnly) {
      if (this._overlay) {
        this._codeMirror.removeOverlay(this._overlay);
      }
      const [start, end] = getIndexPosition(this.props.value, [
        this.props.overlayStart,
        this.props.overlayEnd,
      ]);
      this._overlay = createOverlay(start, end);
      this._codeMirror.addOverlay(this._overlay);
    }
  }