Session establishment hangs with OS X 10.11 client and FreeBSD server

My boss at $dayjob had this issue, so I spent a while on it:

On his OS X 10.11.1 MBP, talking to a FreeBSD 9.3 or 10.3 server, Homebrew's mosh 1.2.6_3 will hang during session startup, most of the time. Examination shows that the mosh script is waiting for ssh, which has a half-closed TCP session (the server has closed its end) and is waiting for Godot. This appears to be a problem with the OS X bundled ssh 6.9p1 client, but is annoyingly hard to debug:

• Only happens on OS X 10.11 (perhaps only on this specific machine). Even running the 10.11 client on OS X 10.12 works fine. But the Homebrew 7.3p1 ssh client also hangs on the 10.11 machine.
• ssh only hangs when run by the Mosh perl script (or my cut-down debug version of it).
• ssh only hangs when talking to a FreeBSD server. OS X and Linux servers are fine.
• Only happens when run with --ssh-pty (the default).
• --experimental-remote-ip does not affect the problem-- it happens whether or not ssh is using Mosh's fake-proxy.

Fortunately, we discovered that using -f in place of -n on the ssh command line solves the problem. @glance, was there a reason to use -n and not -f here?

Start by inspecting the Mosh Perl script's SSH invocation, especially the default --ssh-pty path and its use of -n. Reproduce the session-startup hang with the OS X 10.11 client and FreeBSD server, compare -n with -f, and verify that the selected behavior no longer leaves SSH waiting during startup.