Mullion

A mullion is a vertical divider between windows.

Granted, mullion is not a software term, but it is related to building windows, ah, that is, glass-filled holes in the walls of large structures, not what Microsoft does when they ‘build Windows’. Consider more similarities between AEC (architecture-engineering-construction) and software. Both have architects. Both have engineers. And, both are involved in construction when they build things, like windows.