transient.c

/* cc transient.c -o transient -lX11 */

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

int main(void) {
  Display *d;
  Window r, f, t = None;
  XSizeHints h;
  XEvent e;

  d = XOpenDisplay(NULL);
  if (!d)
    exit(1);

  r = DefaultRootWindow(d);
  f = XCreateSimpleWindow(d, r, 100, 100, 400, 400, 0, 0, 0);
  h.min_width = h.max_width = h.min_height = h.max_height = 400;
  h.flags = PMinSize | PMaxSize;
  XSetWMNormalHints(d, f, &h);
  XStoreName(d, f, "floating");
  XMapWindow(d, f);

  XSelectInput(d, f, ExposureMask);
  while (1) {
    XNextEvent(d, &e);
    if (t == None) {
      sleep(5);
      t = XCreateSimpleWindow(d, r, 50, 50, 100, 100, 0, 0, 0);
      XSetTransientForHint(d, t, f);
      XStoreName(d, t, "transient");
      XMapWindow(d, t);
      XSelectInput(d, t, ExposureMask);
    }
  }

  XCloseDisplay(d);
  exit(0);
}