docs: void showPreview( String path ) {
          synchronized (lockPreview) {
              if( path == null ) {
                  pendingPreview = null;
              } else if( previewThread == null ) {
                  pendingPreview = path;
                  previewThread = new PreviewThread();
                  previewThread.start();
              } else {
                  pendingPreview = path;
              }
          }
      }