bodyDataLayer = new DataLayer(rowDataProvider); // rowDataProvider - custom created row provider

final HoverLayer hoverLayer = new HoverLayer(bodyDataLayer);

selectionLayer = new SelectionLayer(hoverLayer);
selectionProvider = new RowSelectionProvider<NatTreeNode>(selectionLayer, rowDataProvider, false); 

treeLayer = new TreeLayer(selectionLayer, new TreeRowModel<NatTreeNode>(rowDataProvider));
			
setUnderlyingLayer(new ViewportLayer(treeLayer));

public class MyViewportLayer extends ViewportLayer {

	private NatTable natTable;
	private GridLayer gridLayer;
	private FixedSummaryRowLayer summaryRowLayer;
	
	public MyViewportLayer(IUniqueIndexLayer underlyingLayer) {
		super(underlyingLayer);
	}
	
	@Override
    public int getHeight() {
		return natTable.getClientArea().height - summaryRowLayer.getHeight() - 
				gridLayer.getColumnHeaderLayer().getHeight();
    }

      
    @Override
    public int getRowPositionByY(int y) {
        return getRowPositionByY(this, y);
    }
    
    private int getRowPositionByY(ILayer layer, int y) {
        int height = super.getHeight();
        if (y < 0) {
            return -1;
        }else if (y>=height){
        	return findRowPosition(0, 0, layer, y, height, super.getRowCount());
        }
        return findRowPosition(0, 0, layer, y, height, layer.getRowCount());
    }

    private int findRowPosition(
            int yOffset, int rowOffset, ILayer layer, int y, int totalHeight, int rowCount) {

        double size = (double) (totalHeight - yOffset) / (rowCount - rowOffset);
        int rowPosition = rowOffset + (int) ((y - yOffset) / size);

        int startY = layer.getStartYOfRowPosition(rowPosition);
        int endY = startY + layer.getRowHeightByPosition(rowPosition);
        if (y < startY) {
            if (startY == totalHeight) {
                return rowCount;
            }
            return findRowPosition(yOffset, rowOffset, layer, y, startY, rowPosition);
        }
        else if (y >= endY) {
            return findRowPosition(endY, rowPosition + 1, layer, y, totalHeight, rowCount);
        }
        else {
            return rowPosition;
        }
    }

	public NatTable getNatTable() {
		return natTable;
	}

	public void setNatTable(NatTable natTable) {
		this.natTable = natTable;
	}

	public GridLayer getGridLayer() {
		return gridLayer;
	}

	public void setGridLayer(GridLayer gridLayer) {
		this.gridLayer = gridLayer;
	}

	public FixedSummaryRowLayer getSummaryRowLayer() {
		return summaryRowLayer;
	}

	public void setSummaryRowLayer(FixedSummaryRowLayer summaryRowLayer) {
		this.summaryRowLayer = summaryRowLayer;
	}
    
}