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

int main() {
    Display *display;
    int screen;
    unsigned long white, black;
    Window mainWindow, focusWindow;
    XWMHints *hints;
    Atom wmProtocols, wmTakeFocus;
    XEvent e;

    display = XOpenDisplay(NULL);
    if (!display) return 1;
    screen = DefaultScreen(display);
    white = WhitePixel(display, screen);
    black = BlackPixel(display, screen);

    mainWindow = XCreateSimpleWindow(display, DefaultRootWindow(display),
                                     100, 100, 300, 200, 0, white, white);
    focusWindow = XCreateSimpleWindow(display, mainWindow,
                                      -1, -1, 1, 1, 0, white, white);

    hints = XAllocWMHints();
    hints->flags = InputHint;
    hints->input = False;
    XSetWMHints(display, mainWindow, hints);

    wmProtocols = XInternAtom(display, "WM_PROTOCOLS", False);
    wmTakeFocus = XInternAtom(display, "WM_TAKE_FOCUS", False);
    XSetWMProtocols(display, mainWindow, &wmTakeFocus, 1);

    XSelectInput(display, focusWindow, FocusChangeMask);

    XMapWindow(display, focusWindow);
    XMapWindow(display, mainWindow);

    for(;;) {
        XNextEvent(display, &e);
        if (e.type == ClientMessage &&
            e.xclient.message_type == wmProtocols &&
            e.xclient.data.l[0] == wmTakeFocus) {
            XSetInputFocus(display, focusWindow, RevertToNone, e.xclient.data.l[1]);
        } else if (e.type == FocusIn) {
            XSetWindowBackground(display, mainWindow, black);
            XClearWindow(display, mainWindow);
        } else if (e.type == FocusOut) {
            XSetWindowBackground(display, mainWindow, white);
            XClearWindow(display, mainWindow);
        }
    }
}