Sealed joints. When installing a window in a tiled shower enclosure, make sure the joint between the jamb and the cementboard is sufficiently sealed with silicone caulk and a waterproofing membrane. The membrane bridging the two materials creates a stable substrate for the tile.
What to do if you have a window in your shower..... Install a vinyl window with privacy glass and wrap it with vinyl trim. Caulk the seams to make it waterproof. Amazing! So much more affordable than glass block!