From 89d4dd8952a1389ba72012a8efb42ee6ed1492d8 Mon Sep 17 00:00:00 2001
From: Neil <4138956+nwparker@users.noreply.github.com>
Date: Sat, 26 Sep 2026 12:53:14 -0700
Subject: [PATCH] perf(terminal): scan synchronized markers in linear work
 (#23015)

---
 .../foreground-output-scan.test.ts            | 80 +++++++++++++++++++
 .../pty-connection/foreground-output-scan.ts  | 21 ++---
 2 files changed, 92 insertions(+), 9 deletions(-)
 create mode 100644 src/renderer/src/components/terminal-pane/pty-connection/foreground-output-scan.test.ts

diff --git a/src/renderer/src/components/terminal-pane/pty-connection/foreground-output-scan.test.ts b/src/renderer/src/components/terminal-pane/pty-connection/foreground-output-scan.test.ts
new file mode 100644
index 00000000000..7d2a280eafc
--- /dev/null
+++ b/src/renderer/src/components/terminal-pane/pty-connection/foreground-output-scan.test.ts
@@ -0,0 +1,80 @@
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import {
+  scanSynchronizedForegroundOutput,
+  SYNCHRONIZED_OUTPUT_START_SEQUENCE as start,
+  SYNCHRONIZED_OUTPUT_END_SEQUENCE as end
+} from './foreground-output-scan'
+
+afterEach(() => vi.restoreAllMocks())
+
+describe('foreground synchronized output scan', () => {
+  it.each([start, end])('bounds searched characters for a flood of %j markers', (marker) => {
+    const data = marker.repeat(8192)
+    const originalIndexOf = String.prototype.indexOf
+    let searchedCharacters = 0
+    const spy = vi.spyOn(String.prototype, 'indexOf').mockImplementation(function (
+      this: string,
+      search: string,
+      position = 0
+    ): number {
+      const result = originalIndexOf.call(this, search, position)
+      if (this === data && (search === start || search === end)) {
+        searchedCharacters += (result < 0 ? data.length : result + search.length) - position
+      }
+      return result
+    })
+    const result = scanSynchronizedForegroundOutput(data, '', false)
+    spy.mockRestore()
+    console.info(JSON.stringify({ markers: 8192, chars: data.length, searchedCharacters }))
+    expect(result).toEqual({
+      started: marker === start,
+      ended: marker === end,
+      active: marker === start,
+      markerTail: data.slice(-7)
+    })
+    expect(searchedCharacters).toBeLessThanOrEqual(data.length * 2)
+  })
+
+  it('preserves the last marker and records both transitions in the current chunk', () => {
+    expect(scanSynchronizedForegroundOutput(`${end}paint${start}`, '', true)).toEqual({
+      started: true,
+      ended: true,
+      active: true,
+      markerTail: start.slice(1)
+    })
+    expect(scanSynchronizedForegroundOutput(`${start}paint${end}`, '', false)).toEqual({
+      started: true,
+      ended: true,
+      active: false,
+      markerTail: end.slice(1)
+    })
+  })
+
+  it.each([start, end])('recognizes %j at every socket split', (marker) => {
+    for (let split = 1; split < marker.length; split++) {
+      const first = scanSynchronizedForegroundOutput(marker.slice(0, split), '', marker === end)
+      const second = scanSynchronizedForegroundOutput(
+        marker.slice(split),
+        first.markerTail,
+        first.active
+      )
+      expect(second).toEqual({
+        started: marker === start,
+        ended: marker === end,
+        active: marker === start,
+        markerTail: marker.slice(1)
+      })
+    }
+  })
+
+  it('leaves the latch unchanged for empty output and malformed lookalikes', () => {
+    for (const data of ['', 'ordinary output', '\x1b[?2026x'.repeat(2048)]) {
+      expect(scanSynchronizedForegroundOutput(data, '', true)).toEqual({
+        started: false,
+        ended: false,
+        active: true,
+        markerTail: data.slice(-7)
+      })
+    }
+  })
+})
diff --git a/src/renderer/src/components/terminal-pane/pty-connection/foreground-output-scan.ts b/src/renderer/src/components/terminal-pane/pty-connection/foreground-output-scan.ts
index 8d8d53eebb6..b74f44a6d09 100644
--- a/src/renderer/src/components/terminal-pane/pty-connection/foreground-output-scan.ts
+++ b/src/renderer/src/components/terminal-pane/pty-connection/foreground-output-scan.ts
@@ -58,27 +58,30 @@ export function scanSynchronizedForegroundOutput(
   let active = wasActive
   let started = false
   let ended = false
-  let offset = 0
+  let startIndex = scanData.indexOf(SYNCHRONIZED_OUTPUT_START_SEQUENCE)
+  let endIndex = scanData.indexOf(SYNCHRONIZED_OUTPUT_END_SEQUENCE)
 
-  while (offset < scanData.length) {
-    const startIndex = scanData.indexOf(SYNCHRONIZED_OUTPUT_START_SEQUENCE, offset)
-    const endIndex = scanData.indexOf(SYNCHRONIZED_OUTPUT_END_SEQUENCE, offset)
-    if (startIndex === -1 && endIndex === -1) {
-      break
-    }
+  // Each marker search advances independently, so a missing counterpart is scanned only once.
+  while (startIndex !== -1 || endIndex !== -1) {
     if (endIndex !== -1 && (startIndex === -1 || endIndex < startIndex)) {
       active = false
       if (endIndex + SYNCHRONIZED_OUTPUT_END_SEQUENCE.length > currentChunkStartIndex) {
         ended = true
       }
-      offset = endIndex + SYNCHRONIZED_OUTPUT_END_SEQUENCE.length
+      endIndex = scanData.indexOf(
+        SYNCHRONIZED_OUTPUT_END_SEQUENCE,
+        endIndex + SYNCHRONIZED_OUTPUT_END_SEQUENCE.length
+      )
       continue
     }
     active = true
     if (startIndex + SYNCHRONIZED_OUTPUT_START_SEQUENCE.length > currentChunkStartIndex) {
       started = true
     }
-    offset = startIndex + SYNCHRONIZED_OUTPUT_START_SEQUENCE.length
+    startIndex = scanData.indexOf(
+      SYNCHRONIZED_OUTPUT_START_SEQUENCE,
+      startIndex + SYNCHRONIZED_OUTPUT_START_SEQUENCE.length
+    )
   }
 
   return {