clutter grid sizing problem

gistfile1.py

import sys
from gi.repository import Clutter

stage_size = (800, 600)

Clutter.init(sys.argv)
stage = Clutter.Stage()
stage.set_size(*stage_size)
stage.set_title('Problems...')


def stage_key(element, event):
    if event.keyval == Clutter.Escape:
        clutter_quit()


def clutter_quit(*args):
    Clutter.main_quit()

stage.connect('destroy', clutter_quit)
stage.connect('key-press-event', stage_key)


def color(string):
    return Clutter.color_from_string(string)[1]

vcontainer = Clutter.Actor()
vcontainer.set_size(*stage.get_size())
vcontainer.set_background_color(color('Dark Magenta'))

vcontainer_layout = Clutter.BoxLayout.new()
vcontainer_layout.set_orientation(Clutter.Orientation.VERTICAL)
vcontainer_layout.set_spacing(2)
vcontainer.set_layout_manager(vcontainer_layout)

header = Clutter.Actor()
header.set_background_color(color('Hot Pink'))
header.set_height(100)
header.set_x_expand(True)

vcontainer.add_child(header)

grid = Clutter.Actor()
grid.set_background_color(color('Orange Red'))
grid.set_y_expand(True)

grid_layout = Clutter.GridLayout.new()
grid_layout.set_column_spacing(2)
grid_layout.set_row_spacing(2)

grid.set_layout_manager(grid_layout)

vcontainer.add_child(grid)

footer = Clutter.Actor()
footer.set_background_color(color('Gold'))
footer.set_height(100)
footer.set_x_expand(True)

vcontainer.add_child(footer)

for i in range(4):
    # fixed size actor 1
    a = Clutter.Actor()
    a.set_background_color(color('Green'))
    a.set_size(50, 50)

    # fixed size actor 2
    b = Clutter.Actor()
    b.set_background_color(color('Yellow'))
    b.set_size(200, 100)

    # compound a + b
    ab = Clutter.Actor()
    ab_layout = Clutter.BoxLayout.new()
    ab_layout.set_spacing(2)
    ab.set_layout_manager(ab_layout)

    ab.add_child(a)
    ab.add_child(b)

    # why does this actor always get a heigh of = a.width + b.width!?
    c = Clutter.Actor()
    c.set_background_color(color('Blue'))
    c.set_height(100)
    c.set_x_expand(True)

    grid_layout.attach(ab, 0, i, 1, 1)
    grid_layout.attach(c, 1, i, 1, 1)

stage.set_user_resizable(True)
stage.add_child(vcontainer)

vcontainer.add_constraint(Clutter.BindConstraint.new(
    stage, Clutter.BindCoordinate.SIZE, 0.0))

stage.show()
Clutter.main()