docs: a subagent that leaves the list closes its terminal

docs/swarm/ui.md

@@ -39,7 +39,9 @@ terminal, and below it the agent's **subagents**: every subagent the swarm
 queue holds terminal rows for from the last 24 hours. Picking one shows its
 terminal in the same preview, and **expand** opens it in its own tab, as it
 does for the agent. The list refreshes every 10 seconds, so a subagent the
-agent spawns shows up once it writes output.
+agent spawns shows up once it writes output. A subagent that drops out of the
+list closes its terminal, and a later subagent with the same name starts
+unpicked.
 
 On a terminal's own page (`/agents/<name>/terminal`, or a subagent's) the
 header has an **N running subagents** badge. It opens a list of the agent's

frontend/packages/swarm-ui/src/agent/terminal/SubagentTerms.tsx

@@ -8,18 +8,27 @@
 // agent's own: subagents take no input.
 //
 // Mount it keyed by agent name: the refresh timer is armed once per mount.
-import { useState } from "preact/hooks";
+import { useEffect, useState } from "preact/hooks";
 import { ApiErrorPanel } from "@hive/shared/api-error-panel.js";
 import { Badge } from "@hive/shared/badge.js";
 import { AgentTermPreview } from "./AgentTermPreview.js";
-import { STALE_S } from "./subagent-list.js";
+import { STALE_S, stillListed } from "./subagent-list.js";
 import { useSubagentList } from "./useSubagentList.js";
 import { RelativeTime } from "../../ui/relative-time/RelativeTime.js";
 import "./SubagentTerms.css";
 
 export function SubagentTerms({ agentName }: { agentName: string }) {
   const { subagents, error } = useSubagentList(agentName);
-  const [selected, setSelected] = useState<string | null>(null);
+  const [picked, setPicked] = useState<string | null>(null);
+  // The refresh can drop the picked subagent (its stream is gone). Only a
+  // name still in the list counts as selected, so the terminal closes with
+  // its badge and a later subagent of the same name does not reopen it.
+  const selected = stillListed(picked, subagents);
+  useEffect(() => {
+    if (picked !== null && subagents !== null && selected === null) {
+      setPicked(null);
+    }
+  }, [picked, subagents, selected]);
 
   // An agent with no subagents shows nothing here — no heading, no
   // "none" line — and so does the first fetch while it is in flight. A
@@ -49,7 +58,7 @@ export function SubagentTerms({ agentName }: { agentName: string }) {
                     ? `hide ${name}'s terminal`
                     : `show ${name}'s terminal`
                 }
-                onClick={() => setSelected(name === selected ? null : name)}
+                onClick={() => setPicked(name === selected ? null : name)}
               />
               {last_row_unix !== null && (
                 <span class="ui-subagent-terms-age">

frontend/packages/swarm-ui/src/agent/terminal/subagent-list.ts

@@ -48,3 +48,18 @@ export function splitRunning(
 export function runningLabel(count: number): string {
   return count === 1 ? "1 running subagent" : `${count} running subagents`;
 }
+
+/**
+ * The picked subagent while the list still holds it, else `null`. The list
+ * refreshes on a timer and can drop a subagent whose stream is gone, so a
+ * pick outlives its badge unless it is checked against the list. `null`
+ * list (not fetched yet) holds no pick either.
+ */
+export function stillListed(
+  picked: string | null,
+  subs: Subagent[] | null,
+): string | null {
+  return picked !== null && subs?.some((s) => s.name === picked)
+    ? picked
+    : null;
+}

frontend/packages/swarm-ui/test/subagent-list.test.mjs

@@ -9,6 +9,7 @@ import {
   isRunning,
   runningLabel,
   splitRunning,
+  stillListed,
 } from "../src/agent/terminal/subagent-list.ts";
 
 const NOW = 1_000_000;
@@ -59,3 +60,12 @@ test("the badge counts in words", () => {
   assert.equal(runningLabel(1), "1 running subagent");
   assert.equal(runningLabel(4), "4 running subagents");
 });
+
+test("a pick holds only while the refreshed list still has it", () => {
+  const subs = [sub("a", 10), sub("b", 20)];
+  assert.equal(stillListed("a", subs), "a");
+  assert.equal(stillListed("gone", subs), null);
+  assert.equal(stillListed("a", []), null);
+  assert.equal(stillListed("a", null), null);
+  assert.equal(stillListed(null, subs), null);
+});