anchor.diff:    bool active = false;
   
     ngOnChanges(Map<String, SimpleChange> changes) {
  -    updatePreview();
  +    if (active) updatePreview();
  +  }
  +
  +  void ngAfterContentInit() {
  +    _htmlDiv ??= querySelector('#previewPane');
     }
   
     void updatePreview() {
  -    _htmlDiv ??= querySelector('#previewPane');
  -
       _htmlDiv.setInnerHtml(textProcessingService.convertMarkdownToHtml(content),
           treeSanitizer: _nullSanitizer);
     }
  
positive.code:     selector: 'markdown-preview',
      templateUrl: 'markdownpreview.html',
      directives: [NgModel, NgStyle, NgClass])
  class MarkdownPreview extends EditorComponentBase implements OnChanges {
    final _nullSanitizer = NullTreeSanitizer();
  
    DivElement _htmlDiv;
  
negative.code:       EventBusService newEventBusService)
        : super(newTextProcessingService, newTextareaDomService, newThemeService,
              newEventBusService) {
      eventBusService.subscribe('ShowMarkdownPreview', () {
        active = true;
        updatePreview();
      });
      eventBusService.subscribe('HideMarkdownPreview', () => active = false);
    }
  
  