kill placeholder windows when the actual window appears

src/manage.c

@@ -327,6 +327,11 @@ void manage_window(xcb_window_t window, xcb_get_window_attributes_cookie_t cooki
     }
 
     DLOG("new container = %p\n", nc);
+    if (nc->window != NULL) {
+        if (!restore_kill_placeholder(nc->window->id)) {
+            DLOG("Uh?! Container without a placeholder, but with a window, has swallowed this to-be-managed window?!\n");
+        }
+    }
     nc->window = cwindow;
     x_reinit(nc);