Various ICCCM compliant window managers require an application to take action upon WM_DELETE_WINDOW in order to exit when the 'X' in the upper right corner is clicked. At least openbox and fvwm perform no action without this patch and the 'q' key must be used to close mupdf.

http://tronche.com/gui/x/icccm/sec-4.html#s-4.2.8.1

diff --git a/apps/x11_main.c b/apps/x11_main.c
index 395f9fd..bd8add7 100644
--- a/apps/x11_main.c
+++ b/apps/x11_main.c
@@ -51,6 +51,8 @@ static Display *xdpy;
 static Atom XA_TARGETS;
 static Atom XA_TIMESTAMP;
 static Atom XA_UTF8_STRING;
+static Atom XA_WM_DELETE_WINDOW;
+static Atom XA_WM_PROTOCOLS;
 static int x11fd;
 static int xscr;
 static Window xwin;
@@ -113,6 +115,8 @@ static void winopen(void)
 	XA_TARGETS = XInternAtom(xdpy, "TARGETS", False);
 	XA_TIMESTAMP = XInternAtom(xdpy, "TIMESTAMP", False);
 	XA_UTF8_STRING = XInternAtom(xdpy, "UTF8_STRING", False);
+	XA_WM_DELETE_WINDOW = XInternAtom(xdpy, "WM_DELETE_WINDOW", False);
+	XA_WM_PROTOCOLS = XInternAtom(xdpy, "WM_PROTOCOLS", False);
 
 	xscr = DefaultScreen(xdpy);
 
@@ -147,6 +151,8 @@ static void winopen(void)
 	XSelectInput(xdpy, xwin,
 		StructureNotifyMask | ExposureMask | KeyPressMask |
 		PointerMotionMask | ButtonPressMask | ButtonReleaseMask);
+	
+	XSetWMProtocols(xdpy, xwin, &XA_WM_DELETE_WINDOW, 1);
 
 	mapped = 0;
 
@@ -544,6 +550,7 @@ int main(int argc, char **argv)
 	int len;
 	char buf[128];
 	KeySym keysym;
+	XClientMessageEvent *cmev;
 	int oldx = 0;
 	int oldy = 0;
 	int resolution = 72;
@@ -619,6 +626,13 @@ int main(int argc, char **argv)
 				dirty = 1;
 				break;
 
+			case ClientMessage:
+				cmev = (XClientMessageEvent *)&xevt;
+				if (cmev->message_type == XA_WM_PROTOCOLS &&
+					cmev->data.l[0] == XA_WM_DELETE_WINDOW)
+					closing = 1;
+				break;
+
 			case ConfigureNotify:
 				if (gapp.image)
 				{