x11/xscope - The NetBSD Packages Collection

XSCOPE is a program to monitor the connections between the X11 window
server and a client program.  xscope runs as a separate process.  By
adjusting the host and/or display number that a X11 client attaches
to, the client is attached to xscope instead of X11.  xscope attaches
to X11 as if it were the client.  All bytes from the client are sent
to xscope which passes them on to X11; all bytes from X11 are sent to
xscope which sends them on to the client.  xscope is transparent to
the client and X11.

In addition to passing characters back and forth, xscope will print
information about this traffic on stdout, giving performance and
debugging information for an X11 client and server.