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Sshd: problem with X11 passthru

I have encountered a problem with the sshd server when the client
specifies X11 passthru i.e. ssh -Y or ssh -X.

The allocation of virtual displays doesn't work correctly if two or
more X11-enabled sessions run at the same time.

As I understand it, the server should give each session a unique
display number under these circumstances:  localhost:10.0,
localhost:11.0, localhost:12.0 etc.

Instead, all the sessions have DISPLAY=localhost:10.0 at the same
time.  This causes a variety of problems - loss of access to the X
server (due to changed Magic Cookie authorization on the server when a
new client connects), or the appearance of windows on the wrong server
(if two different clients connect).

(Note that setting up Magic Cookie authorization on the client machine
makes no difference to this bug.  It prevents the client-side warning
about 'fake authorization' but it doesn't cure any of the server-side

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