anchor.diff:      // designated by `targetNodeUID`.
     }
   
  +  /*
  +   * Drawing the drag image.
  +   */
  +
  +  // Constants to draw the drag image.
  +
  +  // Font for the filenames in the stack.
  +  final String stackItemFontName = '15px Helvetica';
  +  // Font for the counter.
  +  final String counterFontName = '12px Helvetica';
  +  // Basic height of an item in the stack.
  +  final int stackItemHeight = 30;
  +  // Stack item radius.
  +  final int stackItemRadius = 15;
  +  // Additional space for shadow.
  +  final int additionalShadowSpace = 10;
  +  // Space between stack and counter.
  +  final int stackCounterSpace = 5;
  +  // Stack item interspace.
  +  final int stackItemInterspace = 3;
  +  // Text padding in the stack item.
  +  final int stackItemPadding = 10;
  +  // Counter padding.
  +  final int counterPadding = 10;
  +  // Counter height.
  +  final int counterHeight = 20;
  +  // Stack item text vertical position
  +  final int stackItemTextPosition = 20;
  +  // Counter text vertical position
  +  final int counterTextPosition = 15;
  +
     TreeViewDragImage treeViewDragImage(TreeView view,
                                         List<String> nodesUIDs,
                                         html.MouseEvent event) {
  
positive.code:     for(int i = placeholderCount ; i >= 0 ; i --) {
        html.Rectangle rect = new html.Rectangle(0.5 + i * stackItemInterspace,
            0.5 + i * stackItemInterspace,
            width + stackItemPadding * 2,
            stackItemHeight);
        roundRect(context,
            rect,
  
negative.code:     }
  
      // No shadows.
      _setShadow(context, 0, 'rgba(0, 0, 0, 0)', 0, 0);
  
      // Draw text in the stack item.
      context.font = stackItemFontName;
      context.setFillColorRgb(0, 0, 0, 1);
      context.fillText(stackLabel, stackItemPadding, stackItemTextPosition);
    }
  
    /**
     * Draw the counter and its bezel.
     *
     */
    void _drawCounter(html.CanvasRenderingContext2D context,
                      int x,
                      int y,
                      int counterWidth,
                      String counterString) {
      // Draw counter bezel.
      context.lineWidth = 3;
      context.setFillColorRgb(128, 128, 255, 1);
      html.Rectangle rect = new html.Rectangle(x,
          y,
          counterWidth + counterPadding * 2,
          counterHeight);
      roundRect(context, rect, radius: 10, fill: true, stroke: false);
  
      // Draw text of counter.
      context.font = counterFontName;
      context.setFillColorRgb(255, 255, 255, 1);
      context.fillText(counterString,
          x + counterPadding,
          y + counterTextPosition);
    }
  
    void _addAllFiles() {
  