Don't know about GLFW, perhaps it's buggy, but X11 fullscreen windows don't work like that. Any window manager worth its salt will force the window to fit the (single, non-virtual) screen.
You want to either bypass the window manager completely (use OverrideRedirect window attribute), or ask your WM to cooperate (use window property _NET_WM_STATE_FULLSCREEN). The first approach has numerous drawbacks, so let's use the second one. The following program will display a window on your display, and then toggle it to the full-screen mode:
#include <X11/X.h>
#include <X11/Xlib.h>
#include <strings.h>
#include <memory.h>
#include <stdlib.h>
#include <stdio.h>
int main ()
{
Display* dis = XOpenDisplay(NULL);
Window win = XCreateSimpleWindow(dis, RootWindow(dis, 0), 0, 0, 10, 10,
0, BlackPixel (dis, 0), BlackPixel(dis, 0));
Atom wm_state = XInternAtom(dis, "_NET_WM_STATE", False);
Atom fullscreen = XInternAtom(dis, "_NET_WM_STATE_FULLSCREEN", False);
XEvent xev;
memset(&xev, 0, sizeof(xev));
xev.type = ClientMessage;
xev.xclient.window = win;
xev.xclient.message_type = wm_state;
xev.xclient.format = 32;
xev.xclient.data.l[0] = 1;
xev.xclient.data.l[1] = fullscreen;
xev.xclient.data.l[2] = 0;
XMapWindow(dis, win);
XSendEvent (dis, DefaultRootWindow(dis), False,
SubstructureRedirectMask | SubstructureNotifyMask, &xev);
XFlush(dis);
/*Sleep 5 seconds before closing.*/
sleep(5);
return(0);
}
You probably to use your real screen dimensions for the window from the start, in order to avoid any resize animation effect.
I didn't try this on a multihead system because I don't have one, but on a single display system it works properly (covers the panel, removes window decorations etc). Please let me know if it works for you.
Update They say for multihead to work, you need to use _NET_WM_FULLSCREEN_MONITORS property (see here). It's an array of 4 integers that should be set like this:
Atom fullmons = XInternAtom(dis, "_NET_WM_FULLSCREEN_MONITORS", False);
XEvent xev;
memset(&xev, 0, sizeof(xev));
xev.type = ClientMessage;
xev.xclient.window = win;
xev.xclient.message_type = fullmons;
xev.xclient.format = 32;
xev.xclient.data.l[0] = 0; /* your topmost monitor number */
xev.xclient.data.l[1] = 0; /* bottommost */
xev.xclient.data.l[2] = 0; /* leftmost */
xev.xclient.data.l[3] = 1; /* rightmost */
xev.xclient.data.l[4] = 0; /* source indication */
XSendEvent (dis, DefaultRootWindow(dis), False,
SubstructureRedirectMask | SubstructureNotifyMask, &xev);
With this, you should be able to set your fullscreen windows to occupy a single monitor, the entire desktop, or (for more than 2 monitors) anything in between.
I have not checked this because I don't have a multihead system.
Best Answer
Although you don't want to use a window manager, you might need to use a window manager.
I haven't dug into the X server sources around this, so I can't definitively say X requires a window manager to run properly. But as somebody who writes X client code, and hacks the X server, on minimalist embedded devices with small screens, low CPU power and no GPU... let's just say, all the major players in that space use one, and have good reasons for it.
If you want to avoid chewing up a lot of disk space, RAM or CPU power doing window management, you should check out matchbox. It's a low-footprint window manager designed to meet those criteria, and it's what many folks in that minimalist embedded space are using. My employer uses it on cell phones, configured so that only one app at a time is visible to the user, and the foreground app takes up the whole screen with no window borders. But you can use it other ways, too - Nokia uses it for their Maemo-based network tablets.