This may get inlined later, but it's nice for now. The focused client if any should always be both at the top of fstack and visible on selmon.
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if (&n->link == &clients)
n = wl_container_of(n->link.next, n, link);
focus(n, n->xdg_surface->surface);
if (&c->link == &clients)
c = wl_container_of(c->link.next, c, link);
focus(c, c->xdg_surface->surface);