this variant is known to work, but focus() is ugly - we need in general a better way to handle multihead, this issel-stuff looks awkward (maybe it might be a good idea to set sel to NULL but to introduce a Client *revert which is set if a screen is unfocused, have to think about it further).

client.c

@@ -81,7 +81,7 @@ void
 focus(Client *c) {
 	Client *old = sel;
 
-	if(!issel || (c && !isvisible(c)))
+	if(c && !isvisible(c))
 		return;
 
 	if(old && old != c) {
@@ -89,6 +89,7 @@ focus(Client *c) {
 		XSetWindowBorder(dpy, old->win, dc.norm[ColBorder]);
 	}
 	if(c) {
+		if(issel) {
 			detachstack(c);
 			c->snext = stack;
 			stack = c;
@@ -97,6 +98,9 @@ focus(Client *c) {
 			XSetInputFocus(dpy, c->win, RevertToPointerRoot, CurrentTime);
 		}
 		else
+			XSetWindowBorder(dpy, c->win, dc.norm[ColBorder]);
+	}
+	else if(issel)
 		XSetInputFocus(dpy, root, RevertToPointerRoot, CurrentTime);
 	sel = c;
 	drawstatus();

event.c

@@ -269,8 +269,8 @@ leavenotify(XEvent *e) {
 	XCrossingEvent *ev = &e->xcrossing;
 
 	if((ev->window == root) && !ev->same_screen) {
-		focus(NULL);
 		issel = False;
+		focus(sel);
 	}
 }