Create a System Tray Icon in Linux without GTK/QT

Question

I need to create a Tray Icon for my application using purely the Xlib. After some searching, reading the XEmbed documentation and some stuff on SO, this is what the code looks like so far (mostly copy & paste). I understand what it does, but not why. It is creating a tray icon appearantly, but only 1px wide in my mate-panels area. What am I doing wrong? Why is it not the size of a normal tray icon?

Here is my code:

#include <X11/Xutil.h>
#include <string.h>

#define MIN(A, B)               ((A) < (B) ? (A) : (B))

/* --------- XEMBED and systray stuff */
#define SYSTEM_TRAY_REQUEST_DOCK   0
#define SYSTEM_TRAY_BEGIN_MESSAGE   1
#define SYSTEM_TRAY_CANCEL_MESSAGE  2

static int trapped_error_code = 0;
static int (*old_error_handler) (Display *, XErrorEvent *);

static int
error_handler(Display     *display, XErrorEvent *error) {
    trapped_error_code = error->error_code;
    return 0;
}

void
trap_errors(void) {
    trapped_error_code = 0;
    old_error_handler = XSetErrorHandler(error_handler);
}

int
untrap_errors(void) {
    XSetErrorHandler(old_error_handler);
    return trapped_error_code;
}

void
send_systray_message(Display* dpy, long message, long data1, long data2, long data3) {
    XEvent ev;

    Atom selection_atom = XInternAtom (dpy,"_NET_SYSTEM_TRAY_S0",False);
    Window tray = XGetSelectionOwner (dpy,selection_atom);

    if ( tray != None)
        XSelectInput (dpy,tray,StructureNotifyMask);

    memset(&ev, 0, sizeof(ev));
    ev.xclient.type = ClientMessage;
    ev.xclient.window = tray;
    ev.xclient.message_type = XInternAtom (dpy, "_NET_SYSTEM_TRAY_OPCODE", False );
    ev.xclient.format = 32;
    ev.xclient.data.l[0] = CurrentTime;
    ev.xclient.data.l[1] = message;
    ev.xclient.data.l[2] = data1; // <--- your window is only here
    ev.xclient.data.l[3] = data2;
    ev.xclient.data.l[4] = data3;

    trap_errors();
    XSendEvent(dpy, tray, False, NoEventMask, &ev);
    XSync(dpy, False);
    usleep(10000);
    if (untrap_errors()) {
        /* Handle errors */
    }
}

/* ------------ Regular X stuff */
int
main(int argc, char **argv) {
    int width, height;
    XWindowAttributes wa;
    XEvent ev;
    Display *dpy;
    int screen;
    Window root, win;

    /* init */
    if (!(dpy=XOpenDisplay(NULL)))
        return 1;
    screen = DefaultScreen(dpy);
    root = RootWindow(dpy, screen);
    if(!XGetWindowAttributes(dpy, root, &wa))
        return 1;
    width = height = MIN(wa.width, wa.height);

    /* create window */
    win = XCreateSimpleWindow(dpy, root, 0, 0, width, height, 0, 0, 0x7F7F997F);

    send_systray_message(dpy, SYSTEM_TRAY_REQUEST_DOCK, win, 0, 0); // pass win only once
    XMapWindow(dpy, win);

    XSync(dpy, False);

    /* run */
    while(1) {
        while(XPending(dpy)) {
            XNextEvent(dpy, &ev); /* just waiting until we error because window closed */
        }
    }
}

I am thankful for everything that will point me in the right direction, even if its just some documentation that I have not found so far.