I've recently upgraded my x server (so that I could upgrade my DRI drivers and still have AIGLX) on a machine that I send my mouse/keyboard to using x2x.  It doesn't (typically) have a keyboard or mouse attached.

Now that the new x server (1.4.99.2, built from git yesterday) is being
used, I can't move any windows on it unless it's running compiz (I've
tried xfwm4 and kwin). I can grab the titlebar and drag, and the mouse
cursor changes like it's moving the window, but the window doesn't
move. If I have compiz running, I have no problems moving windows.  If
I throw a mouse onto the machine to test with, I can move windows in
kwin and xfwm4.  If I leave the mouse attached, and go back to x2x, I
still can't move windows.  I can't even move windows using the
keybindings for the window managers.

This happens on two machines (though I suspect it would happen on all
my machines, if I used x2x to access them), one with a radeon 9800 and
one with an intel 845, so it doesn't appear to be a driver issue.

In fact, it does work with metacity as well...  With metacity, the window doesn't move till I actually stop moving the mouse (though I don't need to release the mouse button...  I can drag, stop, and the window moves, and then drag again, stop, and the window moves).  I've tested as well on my laptop (first generation radeon mobility), and see the same problem.  Of course, as
soon as I use the touchpad (or USB mouse) to move the windows in metacity, it goes back to showing the content of the windows as I move them.