drag/drop something that follows the X cursor perfectly

asdf.c

#include <GL/glx.h>
#include <stdio.h>
#include <stdlib.h>

struct asdf {
	Display *dpy;
	Window win, root;
	GLXFBConfig *fbc;
	XVisualInfo *vi;
	XSetWindowAttributes swa;
	XWindowAttributes gwa;
	GLXContext gc;

	int w, h;
	int bw, bh, bx, by, px, py;
	int drag;
};

void resize(struct asdf *a, int w, int h)
{
	glViewport(0, 0, w, h);
	glClear(GL_COLOR_BUFFER_BIT);
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	glOrtho(0.0, w, 0.0, h, -1.0, 1.0);
	a->by += h - a->h;
	a->py += h - a->h;
	a->w = w;
	a->h = h;
}

void init(struct asdf *a, int w, int h)
{
	a->dpy = XOpenDisplay(NULL);
	if (a->dpy == NULL) {
		printf("Could not open display");
		exit(1);
	}

	int att[] = {
		GLX_DRAWABLE_TYPE, GLX_WINDOW_BIT,
		GLX_RENDER_TYPE, GLX_RGBA_BIT,
		None
	};
	int n;
	a->fbc = glXChooseFBConfig(a->dpy, DefaultScreen(a->dpy), att, &n);
	a->vi = glXGetVisualFromFBConfig(a->dpy, a->fbc[0]);

	a->root = DefaultRootWindow(a->dpy);
	a->swa.colormap =
	    XCreateColormap(a->dpy, a->root, a->vi->visual, AllocNone);
	a->swa.event_mask = ExposureMask | ButtonPressMask | Button1MotionMask;

	a->win =
	    XCreateWindow(a->dpy, a->root, 0, 0, w, h, 0, a->vi->depth,
			  InputOutput, a->vi->visual, CWColormap | CWEventMask,
			  &(a->swa));
	XMapWindow(a->dpy, a->win);

	a->gc = glXCreateContext(a->dpy, a->vi, NULL, GL_TRUE);
	glXMakeCurrent(a->dpy, a->win, a->gc);

	glClearColor(0.0, 0.0, 0.0, 0.0);
	resize(a, w, h);

	a->bw = 40, a->bh = 40;
	a->bx = 400, a->by = 300;
	a->px = a->bx, a->py = a->by;
	a->drag = 0;
}

void quit(struct asdf *a)
{
	glXMakeCurrent(a->dpy, None, NULL);
	glXDestroyContext(a->dpy, a->gc);
	XDestroyWindow(a->dpy, a->win);
	XCloseDisplay(a->dpy);
	exit(0);
}

void ev_handle(struct asdf *a)
{
	XEvent e;
	while (XPending(a->dpy)) {
		XNextEvent(a->dpy, &e);
		switch (e.type) {
		case Expose:
			XGetWindowAttributes(a->dpy, a->win, &(a->gwa));
			if (a->w != a->gwa.width || a->h != a->gwa.height)
				resize(a, a->gwa.width, a->gwa.height);
			break;
		case MotionNotify:
			if (!a->drag)
				break;
			if (e.xmotion.x < 0)
				e.xmotion.x = 0;
			else if (e.xmotion.x > a->w - 1)
				e.xmotion.x = a->w - 1;
			if (e.xmotion.y < 0)
				e.xmotion.y = 0;
			else if (e.xmotion.y > a->h - 1)
				e.xmotion.y = a->h - 1;
			a->bx += e.xmotion.x - a->px;
			a->by -= e.xmotion.y - a->py;
			a->px = e.xmotion.x;
			a->py = e.xmotion.y;
			break;
		case ButtonPress:
			if (e.xbutton.button != 1)
				quit(a);
			if (e.xbutton.x - a->bx <= a->bw
			    && e.xbutton.x - a->bx >= 0
			    && a->h - e.xbutton.y - a->by <= a->bh
			    && a->h - e.xbutton.y - a->by >= 0) {
				a->px = e.xbutton.x;
				a->py = e.xbutton.y;
				a->drag = 1;
			} else {
				a->drag = 0;
			}
			break;
		default:
			break;
		}
	}
}

void draw(struct asdf *a)
{
	glColor3f(0.0, 1.0, 0.0);
	glBegin(GL_LINE_LOOP);
	glVertex3f(a->bx + 0.5, a->by - 0.5, 0.0);
	glVertex3f(a->bx + a->bw + 0.5, a->by - 0.5, 0.0);
	glVertex3f(a->bx + a->bw + 0.5, a->by + a->bh - 0.5, 0.0);
	glVertex3f(a->bx + 0.5, a->by + a->bh - 0.5, 0.0);
	glEnd();
	glFlush();
	glClear(GL_COLOR_BUFFER_BIT);
}

int main(void)
{
	struct asdf a;
	init(&a, 800, 600);
#define get_t PFNGLXGETVIDEOSYNCSGIPROC
#define wait_t PFNGLXWAITVIDEOSYNCSGIPROC
	get_t get_vsync = (get_t) glXGetProcAddress((const GLubyte *)"glXGetVideoSyncSGI");
	wait_t wait_vsync = (wait_t) glXGetProcAddress((const GLubyte *)"glXWaitVideoSyncSGI");
	unsigned vsc;

	while (1) {
		ev_handle(&a);
		draw(&a);
		get_vsync(&vsc);
		wait_vsync(2, (vsc + 1) % 2, &vsc);
	}
}

Makefile

all:
	gcc -O3 asdf.c -o asdf -lGL