nx-x11proto-xext-dev binary package in Ubuntu Mantic amd64

 NX is a software suite which implements very efficient
 compression of the X11 protocol. This increases performance when
 using X applications over a network, especially a slow one.
 .
 This package provides development headers describing the wire protocol
 for various extensions, the client-side libraries of which are provided
 in the Xext library.

Source package