fix(tui): make color-scheme detection fully covered and race-free

The color-scheme detection block left packages/ui/tui/src/index.ts below
the 100% per-file coverage gate on three counts: the .then callback's
`scheme === undefined` branch was reachable only via the 2s query
timeout, the .catch only via a query-write failure, and the
`editor.borderColor` assignment inside applyColorScheme was dead code —
the next line's setStatus() immediately reassigns editor.borderColor.

Register the scheme listener before firing the startup query so the
query's own reply is delivered through the listener (the same path as
later theme switches), which removes the redundant .then re-application
and its uncoverable undefined branch, and closes the theoretical window
where a synchronous reply lands before the listener exists. Drop the
dead editor.borderColor line. Cover the rest: a same-scheme report
(early return) and a terminal that throws on the query write (the
swallowed .catch).

packages/ui/tui/src/index.ts

@@ -1134,25 +1134,24 @@ export function createTuiChat(
     currentScheme = scheme
     Object.assign(palette, createPalette(resolved.color, scheme))
     Object.assign(mdTheme, markdownTheme(palette))
-    editor.borderColor = text => palette.dim(text)
     rebuildTranscript(false)
     setStatus(agent.status)
     requestRender()
   }
   let currentScheme: TerminalColorScheme = 'dark'
 
-  // Detect the terminal's color scheme via device-status report. Most terminals
-  // do not respond, so the promise settles with `undefined` and we keep the
-  // dark-optimised palette.
-  ui.queryTerminalColorScheme({ timeoutMs: 2000 }).then((scheme) => {
-    if (scheme !== undefined) applyColorScheme(scheme)
-  }).catch(() => {
-    // Timeout or query failure — keep dark default.
-  })
-
-  // Live-update when the user switches their terminal theme behind us.
+  // Apply any color scheme the terminal reports. Registering before the query
+  // below means even a synchronous reply reaches `applyColorScheme`; in practice
+  // the startup query's reply is the only report, since dsh-tui leaves
+  // unsolicited color-scheme notifications disabled.
   const disposeSchemeListener = ui.onTerminalColorSchemeChange(applyColorScheme)
 
+  // Ask the terminal for its color scheme via device-status report; the reply,
+  // if any, arrives through the listener above. Most terminals do not respond,
+  // so we keep the dark-optimised palette. Swallow a query-write failure for the
+  // same reason.
+  ui.queryTerminalColorScheme({ timeoutMs: 2000 }).catch(() => {})
+
   const toggleTools = (): void => {
     toolsExpanded = !toolsExpanded
     for (const card of allToolCards) card.setExpanded(toolsExpanded)

packages/ui/tui/tests/tui.spec.ts

@@ -942,6 +942,13 @@ describe('terminal mounting', () => {
     // Initial render uses dark-optimised palette: SGR 2 (dim) for dim text.
     expect(result.terminal.output).toContain('\x1b[2mdeepseek-v4-flash')
 
+    // A report matching the current scheme is a no-op: no palette rebuild or
+    // re-render (ESC [?997;1n = dark, the startup default).
+    const beforeSameScheme = result.terminal.output.length
+    result.terminal.send('\x1b[?997;1n')
+    await tick()
+    expect(result.terminal.output.length).toBe(beforeSameScheme)
+
     // Simulate the terminal responding with a light color scheme report
     // (ESC [?997;2n = light, ESC [?997;1n = dark).
     result.terminal.send('\x1b[?997;2n')
@@ -963,4 +970,23 @@ describe('terminal mounting', () => {
     expect(result.terminal.output).toContain('\x1b[2mdeepseek-v4-flash')
     await dispose(result)
   })
+
+  it('keeps the dark palette when the terminal rejects the color-scheme query', async () => {
+    class QueryFailTerminal extends FakeTerminal {
+      override write(data: string): void {
+        // The device-status query is the only write that fails; the promise
+        // rejects and the swallowed `.catch` leaves the dark palette in place.
+        if (data === '\x1b[?996n') throw new Error('query write failed')
+        super.write(data)
+      }
+    }
+    const terminal = new QueryFailTerminal()
+    const result = await createTuiTestHarness(terminal, vi.fn(), {
+      config: { color: true },
+      cwd: process.cwd(),
+    })
+    await tick()
+    expect(terminal.output).toContain('\x1b[2mdeepseek-v4-flash')
+    await disposeTuiTestHarness(result)
+  })
 })