--- a/nested-client.c
+++ b/nested-client.c
@@ -252,7 +252,26 @@
         if (fds[1].revents & POLLIN) {
             char cmd[64];
             if (fgets(cmd,sizeof(cmd),stdin)) {
-                if (!strncmp(cmd,"close_b",7)) { destroy_popup(&b); fprintf(stderr,"COMMAND close_b DONE\n"); }
+                /* WHY: isolate client content from scene geometry and input region.
+                 * CONTRACT: same toplevel size/position; only private headless client.
+                 * INVARIANT: region updates carry no new client buffer. */
+                if (!strncmp(cmd,"recolor_top",11)) {
+                    top.color = 0x00d020c0;
+                    draw(&top);
+                    fprintf(stderr,"COMMAND recolor_top DONE color=%08x wl=%u\n",
+                        top.color, wl_proxy_get_id((struct wl_proxy *)top.wl));
+                } else if (!strncmp(cmd,"region_empty_top",16) ||
+                           !strncmp(cmd,"region_full_top",15)) {
+                    bool empty = !strncmp(cmd,"region_empty_top",16);
+                    struct wl_region *region = wl_compositor_create_region(compositor);
+                    if (!empty) wl_region_add(region, 0, 0, top.width, top.height);
+                    wl_surface_set_input_region(top.wl, region);
+                    wl_surface_commit(top.wl);
+                    wl_region_destroy(region);
+                    fprintf(stderr,"COMMAND region_%s_top DONE wl=%u\n",
+                        empty ? "empty" : "full",
+                        wl_proxy_get_id((struct wl_proxy *)top.wl));
+                } else if (!strncmp(cmd,"close_b",7)) { destroy_popup(&b); fprintf(stderr,"COMMAND close_b DONE\n"); }
                 else if (!strncmp(cmd,"close_a",7)) { destroy_popup(&b); destroy_popup(&a); fprintf(stderr,"COMMAND close_a DONE\n"); }
                 else if (!strncmp(cmd,"close_owner",11)) { fprintf(stderr,"COMMAND close_owner\n"); running=0; }
             }