Terminal: a shell that ends while its panel is shown again is not replaced

Showing the panel connects a frame later (the panel needs its width first). A shell that ended
before that frame -- reopened and closed by the server within one frame, as headless CI's slow
frames made happen -- was quietly replaced by a new one, against "no reconnect of its own".
The frame now connects only if no shell was open as the showing began. The cause of the
B3 failures on GitHub since 94e903e; a test makes it happen every time.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/ui/run.mjs

@@ -529,6 +529,25 @@ test("B3: an open agent chat opens its own shell from the header; Send to chat f
   assert.deepEqual(p.errors, [])
 })
 
+test("B3: a shell that ends while its panel is being shown again is not replaced by a new one", async () => {
+  const p = await page(b3())
+  await openChatRow(p, "chat2")
+  await p.click('#chat-tools [data-terminal-toggle]')
+  await p.waitForFunction(() => window.__mock.terminals().length === 1)
+  await p.waitForFunction(() => document.querySelector("#terminal-message").textContent === "Connected.")
+  await p.click("#terminal-close")
+  // Shown again, and the shell ends before the panel has drawn its first frame.
+  await p.evaluate(() => {
+    document.querySelector("#chat-tools [data-terminal-toggle]").click()
+    window.__mock.shell("control", { t: "closed", message: "The shell exited." })
+  })
+  await p.evaluate(() => new Promise((r) => requestAnimationFrame(() => requestAnimationFrame(r))))
+  await p.waitForTimeout(200)
+  assert.equal(await p.evaluate(() => window.__mock.terminals().length), 1, "no new shell in its place")
+  assert.equal(await p.textContent("#terminal-message"), "The shell exited.")
+  assert.deepEqual(p.errors, [])
+})
+
 test("B3: the terminal and the activity panel take turns in the panel column", async () => {
   const p = await page(b3({ "GET /chats/chat2/activity": { activity: {} } }))
   await openChatRow(p, "chat2")