Graphical console in stable/10 stuck on heavy writing

[Mark Martinec]

FreeBSD amd64, recent stable/10:

When running some task on a graphical console window
and that task writes lots of output to a console, the
console gets stuck after some time (like 20 minutes),
output stops, keyboard appears unresponsive, alt-ctrl-Fn
is unable to switch consoles - but the machine seems
otherwise sane: I can log in over ssh, file system and
processes are healthy, CPU not busy.

This was first noticed trying to rebuild kde from
ports in a console window. Same happened when bulk
updating packages from a remote site and the pkg update
was busily writing its output to a console.

The same heavy tasks (e.g. ports rebuild) can run
just fine in an ssh session, or in a konsole / xterm.

As a final proof, I tried a perl one-liner which
writes a line (a count and some text) ten times per
second: it manages to screw up the console window
in about 6 minutes.

Interestingly, while the console output is stuck
and I blindly type a command on a keyboard (no echo),
then run a shutdown from another (ssh) session,
the console output gets unstuck during shutdown,
rapidly displays the missing lines, including the
echo of a command typed blindly previously, along
with its output.

Btw, this never happened on this machine while it was
running 9.2 or older versions.

Anybody seen this or can reproduce it?

   Mark