Comment #2 on issue 577 by tudor.gi...@gmail.com: ChildrenShape does not get centered http://code.google.com/p/moose-technology/issues/detail?id=577
http://scg.unibe.ch/archive/masters/Meye06b.pdf - page 29-30
"The keyword fill indicates that a shape should take up all available space in a cell even if the width or height of the cell is larger than the shapes preferred width or height, with pref we tell the layout manager that the shapes preferred size should be considered if possible and with grow we specify that a row or column should take up all the available space that is left at the end of the layouting process."