In mx/mx-grid.c:mx_grid_do_allocate(), this
if (natural_width > priv->max_extent_a)
priv->max_extent_a = natural_width;
if (natural_height > priv->max_extent_b)
priv->max_extent_b = natural_width;
appears to be a cut-and-paste error. The second "natural_width" should be "natural_height".
Downstream bug: https://bugzilla.redhat.com/show_bug.cgi?id=919028