When my SDL window gets a resize event from the window
manager, I have to (if I understand it correctly) make a call
to set_video_mode with the new resolution. It seems that
this in turn sends some event to the window manager.

On some window managers, this causes problems.
This happens, when the window resize is done by a mouse
drag after clicking on the window corner. It only happens
with window managers that attempt to resize continously
throughout the dragging and then not with all of them.

With fvwm and qvwm, the problem is that the window
manager remembers the resize events from set_video_mode
and feeds them back to the SDL window after the dragging
is complete. (Or so I believe.)
With jwm, it is impossible to resize SDL windows.

I assume it is a design flaw in SDL, that set_video_mode
is needed to *respond* to a resize event.

OS: Debian etch on an AMD64
SDL version: 1.2.11, according to Debian

Here is a sample ocaml program that shows the bug.
(But I assume you don't need it.)

;;

Sdl.init_subsystem [`VIDEO];
Sdlevent.enable_events Sdlevent.videoresize_mask;

let window = ref (Sdlvideo.set_video_mode 1200 900 [`RESIZABLE])  in

while true do
  Sdlevent.pump ();
  match Sdlevent.poll () with
  | Some Sdlevent.VIDEORESIZE (w,h) ->
    window := Sdlvideo.set_video_mode w h [`RESIZABLE]
  | _ -> ();
done