How to use the birdears.urwid.decoration.calculate_top_bottom_filler function in birdears

if self.width_type == PACK:
            width, height = self.top_w.pack((),focus=focus)
            if not height:
                raise OverlayError("fixed widget must have a height")
            left, right = calculate_left_right_padding(maxcol,
                self.align_type, self.align_amount, CLIP, width,
                None, self.left, self.right)
        else:
            left, right = calculate_left_right_padding(maxcol,
                self.align_type, self.align_amount,
                self.width_type, self.width_amount,
                self.min_width, self.left, self.right)

        if height:
            # top_w is a fixed widget
            top, bottom = calculate_top_bottom_filler(maxrow,
                self.valign_type, self.valign_amount,
                GIVEN, height, None, self.top, self.bottom)
            if maxrow-top-bottom < height:
                bottom = maxrow-top-height
        elif self.height_type == PACK:
            # top_w is a flow widget
            height = self.top_w.rows((maxcol,),focus=focus)
            top, bottom =  calculate_top_bottom_filler(maxrow,
                self.valign_type, self.valign_amount,
                GIVEN, height, None, self.top, self.bottom)
            if height > maxrow: # flow widget rendered too large
                bottom = maxrow - height
        else:
            top, bottom = calculate_top_bottom_filler(maxrow,
                self.valign_type, self.valign_amount,
                self.height_type, self.height_amount,

def _set_focus_valign_complete(self, size, focus):
        """
        Finish setting the offset and inset now that we have have a
        maxcol & maxrow.
        """
        (maxcol, maxrow) = size
        vt,va = self.set_focus_valign_pending
        self.set_focus_valign_pending = None
        self.set_focus_pending = None

        focus_widget, focus_pos = self._body.get_focus()
        if focus_widget is None:
            return

        rows = focus_widget.rows((maxcol,), focus)
        rtop, rbot = calculate_top_bottom_filler(maxrow,
            vt, va, GIVEN, rows, None, 0, 0)

        self.shift_focus((maxcol, maxrow), rtop)

left, right = calculate_left_right_padding(maxcol,
                self.align_type, self.align_amount,
                self.width_type, self.width_amount,
                self.min_width, self.left, self.right)

        if height:
            # top_w is a fixed widget
            top, bottom = calculate_top_bottom_filler(maxrow,
                self.valign_type, self.valign_amount,
                GIVEN, height, None, self.top, self.bottom)
            if maxrow-top-bottom < height:
                bottom = maxrow-top-height
        elif self.height_type == PACK:
            # top_w is a flow widget
            height = self.top_w.rows((maxcol,),focus=focus)
            top, bottom =  calculate_top_bottom_filler(maxrow,
                self.valign_type, self.valign_amount,
                GIVEN, height, None, self.top, self.bottom)
            if height > maxrow: # flow widget rendered too large
                bottom = maxrow - height
        else:
            top, bottom = calculate_top_bottom_filler(maxrow,
                self.valign_type, self.valign_amount,
                self.height_type, self.height_amount,
                self.min_height, self.top, self.bottom)
        return left, right, top, bottom

# top_w is a fixed widget
            top, bottom = calculate_top_bottom_filler(maxrow,
                self.valign_type, self.valign_amount,
                GIVEN, height, None, self.top, self.bottom)
            if maxrow-top-bottom < height:
                bottom = maxrow-top-height
        elif self.height_type == PACK:
            # top_w is a flow widget
            height = self.top_w.rows((maxcol,),focus=focus)
            top, bottom =  calculate_top_bottom_filler(maxrow,
                self.valign_type, self.valign_amount,
                GIVEN, height, None, self.top, self.bottom)
            if height > maxrow: # flow widget rendered too large
                bottom = maxrow - height
        else:
            top, bottom = calculate_top_bottom_filler(maxrow,
                self.valign_type, self.valign_amount,
                self.height_type, self.height_amount,
                self.min_height, self.top, self.bottom)
        return left, right, top, bottom