The packer normally computes how large a master must be to
just exactly meet the needs of its slaves, and it sets the
requested width and height of the master to these dimensions.
This causes geometry information to propagate up through a
window hierarchy to a top-level window so that the entire
sub-tree sizes itself to fit the needs of the leaf windows.
However, the pack propagate command may be used to
turn off propagation for one or more masters.
If propagation is disabled then the packer will not set
the requested width and height of the packer.
This may be useful if, for example, you wish for a master
window to have a fixed size that you specify.