Comment # 4 on bug 1219177 from Stefan Hundhammer

Created attachment 872190 [details]
Screenshot showing the VStretch

Using Ctrl-Shift-Alt-Y to open the YWidgetSpy, you can instpect the widget
hierarchy and modify widget properties.

You can see here that it uses a VStretch placeholder which (as the name
implies) is vertically stretchable, so it eats the extra space; it is
distributed between the content widget (the table) and that VStretch, so each
of them gets the same proportion of extra space if there is any.

Simply replace this VStretch with a VSpacing(1) or a VSpacing(2) to get some
empty space, but not that much.