left pane shorter than content pane when setting height

[squarewave]

whenever i set the height of the grid, and left column is frozen, the height of the panes is not even. (see attached) 

config: 

          {
                columns: treeListColumns,
                height: screen.height - 500,
                dataSource: treeListDataSource,
                columnMenu: true,
                sortable: true,
                resizable: true,
                selectable: true,
                filterable: true
            }