menu bugs with huge display (xinerama)

Solaris 8, openwin X11 server, CDE window manager
(dtwm)
I'm running X in "Xinerama" mode with one logical
screen
across two physical displays. To X clients, it appears
as if there is one single 2560x1024 display.

Both of the problems below occur when a Tk window is
moved
to the right half of the screen (into the second
monitor), but it
doesn't happen exactly at the same spot each time.
Sometimes
I can get a window fully into the right monitor, plus
another
20 or 30 pixels without problems, but then if I move it
farther to
the right, the problems start.

First, menus (at least within a menubar) become very
slow
and unresponsive. They don't respond at all to
press-and-drag
menu selection. Once started, this problem persists
regardless
of where the window is posistioned.

Second when submenus of a menu are posted, they appear
at
the correct y, but far to the left, in the first
monitor. This problem
eventually goes away if the window is moved to the left
again,
but also not at a consistent x-position.

I checked winfo: x, y, rootx, rooty, screenwidth,
screenheight,
pointerx, and pointery all return values that you'd
expect for the
main window regardless of where the window is. However
when
I post a menu with ".mb1.menu1 post 1800 120", and the
immediately do "winfo rootx .mb1.menu1", it returns
520, which is
1800-1280, and 1280 is half the screen width, one
monitor width.
The menu still appears in the right half of the screen
though. If I
then call ".mb1.menu1 postcascade 2", the sub menu
winds up
(not surprisingly) at an x of 639, both visibly and as
reported
by winfo rootx. (winfo x reports the same as winfo
rootx in these
two cases also.)

dtwm uses a "rooms" virtual window paradigm, so vrootx
and
vrooty should always be zero, and they are reported as
such.
vrootwidth and vrootheight are also reported correctly
by
winfo.