I use the fvwm window manager inside X and am trying to move my
desktop over to vnc. My goal is to move  my "real" desktop inside of
Xvnc and have the ordinary X server just run vncviewer full screen. In
fvwm I have a grid of 9 virtual desktops that I can switch among by
simply moving the mouse off the edge of the current screen. Fvwm then
switches me to the next screen and moves my mouse to the other side.
(If I go off the right side of the screen then I am switched to the
next desktop over and my mouse appears on the left).

Now to the problem: when using vnc I switch to the next desktop but my
mouse does not move back to the opposite side of the screen. This is
very annoying. Presumably the vncserver needs to somehow tell the
viewer that it should move the mouse to some other part of the screen.

How do I fix this problem? I'm looking for an easy solution so "It's
not possible" and "download the source" are not acceptable answers.

It's NOT possible because VNC client overrides the serverside mouse
position. So yes, when you scroll off to the right, and the mouse
flicks to the left, your client instantly says, "NO the mouse is on
the right", and the mouse gets put back. I've used VNC for a long
time, and my workaround is just to set up a hotkey to switch desktops
and use that, or just use the desktop selector to choose which desktop
you want to use.

skermit-ga

VNC protocol does not allow mouse warping. Some nice extensions exist,
but you are not interested in downloading anything.

I use VNC/fvwm everyday. Instead of using the mouse to switch
desktops, I use keyboard shortcuts. My .fvwmrc binds Alt-Shift-arrow
to move to desktops up/down/left/right. You can also make it move a
fraction of one screen. It's mush faster than using the mouse anyway.

Here are the related lines in my .fvwmrc:

# press arrow + meta + shift anywhere, and scroll by 1 page
Key Left	A	SM	Scroll -100 0
Key Right	A	SM	Scroll +100 +0
Key Up		A	SM	Scroll +0   -100
Key Down	A	SM	Scroll +0   +100

# press arrow + meta + crtl, and scroll by 1/10 of a page
Key Left	A	CM	Scroll -10 +0
Key Right	A	CM	Scroll +10 +0
Key Up		A	CM	Scroll +0   -10
Key Down	A	CM	Scroll +0   +10

this is all within the program you cannot change that if you need to
change the way the program react then you need to contact the
programmer. you personnally cannot change the way the program eacts to
your actions