Add -B to new-pane/split-window to block until the command exits, like

run-shell -b.

server.c

@@ -491,6 +491,8 @@ server_child_exited(pid_t pid, int status)
 				log_debug("%%%u exited", wp->id);
 				wp->flags |= PANE_EXITED;
 
+				window_pane_block_finish(wp);
+
 				if (window_pane_destroy_ready(wp))
 					server_destroy_pane(wp, 1);
 				break;