Count visible group headers in a datawindow

In a grid datawindow, i want to get the Y position just after a given row (to display a detail object just below the row).

I did this function :

public function integer of_y_from_row (long al_row);

// Get Y from row <al_row>
Integer          li_Header, li_RowHeight

// Header height
li_Header = Integer(This.idw.Object.Datawindow.Header.Height)

// Detail height
li_RowHeight = Integer(This.idw.Object.Datawindow.Detail.Height)

// Ignore hidden rows (scroll)
al_Row -= (Long(This.idw.Object.DataWindow.FirstRowOnPage) - 1)

// Sum header height, visible rows height and one pixel
RETURN PixelsToUnits(UnitsToPixels(li_Header + (al_Row * li_RowHeight), YUnitsToPixels!) + 1, YPixelsToUnits!)

end function

It works very well, except if there are groups with headers.

Someone have an idea on how to count the number of group headers that are visible before a given row ?

Or any easier way to compute this Y coordinate.

Thanks