perf(terminal): bound the reattach payload agent-signal scan (#12681)

hasCursorAgentReattachPayloadScreenSignal built a char-by-char copy of the
entire reattach payload so it could read the last header plus 5000 chars. On a
2MB daemon snapshot that cost 17.5ms of synchronous renderer main-thread work —
~75% of what xterm then spends parsing the same bytes — and the miss case paid
it in full for a result that is always false.

Two changes, both matching existing in-tree precedent: bound the scan to a
256KB tail (as the kitty tracker already bounds its own scan), and strip via
the shared precompiled CSI_SEQUENCE_PATTERN instead of a hand-rolled loop,
which is also faster in V8 because it copies spans rather than building a rope
per character.

  2MB snapshot, header hit   17.5ms -> 0.80ms  (22x)
  2MB snapshot, miss          8.7ms -> 0.52ms  (17x)
  200KB snapshot, header hit  1.5ms -> 0.62ms  (2.4x)

config/scripts/terminal-reattach-payload-scan-benchmark.mjs reproduces this and
asserts every candidate agrees with the baseline before timing it. It also
records a negative result: porting the daemon mouse mirror's includes()
pre-filter to the kitty tracker makes reattach slower, because snapshots always
contain the introducer.

Adds guards for the two behaviours a future shortcut would silently break: a
CSI-split header must still match, and a header behind the tail bound must not.
Also byte-pins POST_REPLAY_REATTACH_RESET_KEEP_MOUSE, which shipped unpinned.

Co-authored-by: Orca <help@stably.ai>

src/renderer/src/components/terminal-pane/pty-connection.test.ts

@@ -16427,6 +16427,81 @@ describe('connectPanePty', () => {
     disposable.dispose()
   })
 
+  // Why pinned: the classifier strips CSI precisely so a styled header still matches. A future
+  // "just lastIndexOf the raw bytes" shortcut would pass every other test and silently break this.
+  it('still detects the Cursor Agent screen when CSI styling splits the header and the marker', async () => {
+    const { connectPanePty } = await import('./pty-connection')
+    enableActiveRuntimeEnvironment()
+    const transport = createMockTransport('remote:env-1@@terminal-1')
+    const capturedReplayCallback: {
+      current: ((data: string, meta?: { clearBeforeReplay?: boolean }) => void) | null
+    } = { current: null }
+    transport.connect.mockImplementation(async ({ callbacks }: { callbacks: ConnectCallbacks }) => {
+      capturedReplayCallback.current = callbacks.onReplayData ?? null
+      return { id: 'remote:env-1@@terminal-1', replay: '' }
+    })
+    transportFactoryQueue.push(transport)
+    setReattachPaneTitle('renamed shell')
+
+    const pane = createPane(1)
+    const textarea = {} as HTMLTextAreaElement
+    configureTerminalFocusMode(pane, textarea)
+    const manager = createManager(1)
+    const deps = createDeps()
+    const disposable = await withMockedDocumentActiveElement(textarea, async () => {
+      const connection = connectPanePty(pane as never, manager as never, deps as never)
+      await flushAsyncTicks(6)
+
+      capturedReplayCallback.current?.(
+        '\x1b[4;3HCursor \x1b[1mAgent\x1b[0m\x1b[9;3H→\x1b[0m Plan, search, build anything'
+      )
+      await flushAsyncTicks(12)
+
+      expect(pane.terminal.write).toHaveBeenCalledWith(
+        POST_REPLAY_LIVE_AGENT_REATTACH_RESET,
+        expect.any(Function)
+      )
+      return connection
+    })
+    disposable.dispose()
+  })
+
+  // Why pinned: the scan reads only a bounded tail, so a header buried behind megabytes of
+  // scrollback describes a finished run, not the current screen.
+  it('ignores a Cursor Agent screen buried beyond the payload scan tail limit', async () => {
+    const { connectPanePty } = await import('./pty-connection')
+    enableActiveRuntimeEnvironment()
+    const transport = createMockTransport('remote:env-1@@terminal-1')
+    const capturedReplayCallback: {
+      current: ((data: string, meta?: { clearBeforeReplay?: boolean }) => void) | null
+    } = { current: null }
+    transport.connect.mockImplementation(async ({ callbacks }: { callbacks: ConnectCallbacks }) => {
+      capturedReplayCallback.current = callbacks.onReplayData ?? null
+      return { id: 'remote:env-1@@terminal-1', replay: '' }
+    })
+    transportFactoryQueue.push(transport)
+    setReattachPaneTitle('renamed shell')
+
+    const pane = createPane(1)
+    const textarea = {} as HTMLTextAreaElement
+    configureTerminalFocusMode(pane, textarea)
+    const manager = createManager(1)
+    const deps = createDeps()
+    const disposable = await withMockedDocumentActiveElement(textarea, async () => {
+      const connection = connectPanePty(pane as never, manager as never, deps as never)
+      await flushAsyncTicks(6)
+
+      capturedReplayCallback.current?.(
+        `${ANSI_POSITIONED_CURSOR_AGENT_REATTACH_SCREEN}${'shell scrollback\r\n'.repeat(20_000)}`
+      )
+      await flushAsyncTicks(12)
+
+      expect(transport.sendInput).not.toHaveBeenCalledWith('\x1b[I')
+      return connection
+    })
+    disposable.dispose()
+  })
+
   it('downgrades a scrollback-only Cursor Agent signal when the parsed viewport shows a shell', async () => {
     const { connectPanePty } = await import('./pty-connection')
     enableActiveRuntimeEnvironment()

src/renderer/src/components/terminal-pane/pty-connection.ts

@@ -115,6 +115,7 @@ import {
   queuePanePtyResizeIfHeld,
   type PanePtyResizeHoldFlushDetail
 } from '@/lib/pane-manager/pane-pty-resize-hold'
+import { CSI_SEQUENCE_PATTERN } from '../../../../shared/ansi-escape-sequences'
 import {
   buildPostReplayLiveAgentReattachReset,
   POST_REPLAY_LIVE_AGENT_SNAPSHOT_RESET,
@@ -469,33 +470,22 @@ function terminalOwnsDomFocus(terminal: TerminalWithFocusMode): boolean {
   return document.activeElement === terminal.textarea
 }
 
-function stripAnsiCsiSequences(data: string): string {
-  let normalized = ''
-  let index = 0
-  while (index < data.length) {
-    if (data.charCodeAt(index) === 0x1b && data[index + 1] === '[') {
-      index += 2
-      while (index < data.length) {
-        const code = data.charCodeAt(index)
-        index += 1
-        if (code >= 0x40 && code <= 0x7e) {
-          break
-        }
-      }
-      continue
-    }
-    normalized += data[index]
-    index += 1
-  }
-  return normalized
-}
-
 const CURSOR_AGENT_REATTACH_HEADER = 'Cursor Agent'
 const CURSOR_AGENT_REATTACH_INPUT_MARKER = '→'
 const CURSOR_AGENT_REATTACH_SCREEN_SIGNAL_MAX_CHARS = 5000
+// Why bounded: reattach payloads reach multiple MB, but every replay puts the current screen last
+// and only the header nearest the end matters. 256KB clears even a fully SGR-styled frame by ~2x,
+// so the cut only ever drops stale scrollback — which would have been rejected anyway.
+const CURSOR_AGENT_REATTACH_SCAN_TAIL_LIMIT_CHARS = 256 * 1024
 
 function hasCursorAgentReattachPayloadScreenSignal(data: string): boolean {
-  const normalized = stripAnsiCsiSequences(data)
+  const tail =
+    data.length > CURSOR_AGENT_REATTACH_SCAN_TAIL_LIMIT_CHARS
+      ? data.slice(-CURSOR_AGENT_REATTACH_SCAN_TAIL_LIMIT_CHARS)
+      : data
+  // Why CSI only: OSC-carried titles must keep counting as a header occurrence, as they did when
+  // this stripped CSI by hand.
+  const normalized = tail.replace(CSI_SEQUENCE_PATTERN, '')
   // Why: anchor on the LAST header occurrence — replay buffers keep scrollback,
   // and an earlier finished run must not classify the current screen.
   const headerIndex = normalized.lastIndexOf(CURSOR_AGENT_REATTACH_HEADER)

src/shared/terminal-mode-reset-profiles.test.ts

@@ -6,6 +6,7 @@ import {
   POST_REPLAY_LIVE_SNAPSHOT_RESET,
   POST_REPLAY_MODE_RESET,
   POST_REPLAY_REATTACH_RESET,
+  POST_REPLAY_REATTACH_RESET_KEEP_MOUSE,
   RESET_MOUSE_REPORTING,
   buildPostReplayLiveAgentReattachReset,
   replayPayloadEndsWithCursorHidden
@@ -33,6 +34,17 @@ describe('terminal mode reset profiles', () => {
     expect(POST_REPLAY_REATTACH_RESET).not.toContain('\x1b[?2004l')
   })
 
+  // Why ?1004l stays: #944 — a hard-killed TUI leaves the daemon emulator on the alternate buffer,
+  // so this profile can reach a plain shell, where armed focus reporting rings BEL on every pane
+  // switch. Dropping it would also make this byte-identical to the live-agent profile.
+  it('pins the live alternate-screen profile, which keeps mouse reporting but not focus', () => {
+    expect(POST_REPLAY_REATTACH_RESET_KEEP_MOUSE).toBe(
+      '\x1b[0 q\x1b[<99u\x1b[=0u\x1b[?25h\x1b[?1004l'
+    )
+    expect(POST_REPLAY_REATTACH_RESET_KEEP_MOUSE).not.toContain(RESET_MOUSE_REPORTING)
+    expect(POST_REPLAY_REATTACH_RESET_KEEP_MOUSE).not.toBe(POST_REPLAY_LIVE_AGENT_REATTACH_RESET)
+  })
+
   // Why: #12101 — a cold-restored seed re-arms mouse reporting for a dead TUI.
   it('disarms mouse reporting on the cold-restore seed', () => {
     expect(COLD_RESTORE_SEED_MODE_RESET).toBe(RESET_MOUSE_REPORTING)