Adding a toolbar to a StackedPanel

I am trying to add a toolbar on top of a stackedpanel widget which contains a datagrid. The toolbar ends up being overlayed by the datagrid. Am I completely misusing the StackedPanel layout? I thought it would layout widget vertically, but it seems to be overlaying them.

    this.toolbar = new Toolbar<Widget>()
    this.toolbar.addItem('connect', new ToolbarButton({
      icon: bugIcon,