fix(terminal): keep desktop history when a hidden pane is revealed (#33534)

Main built its copy of a hidden pane from the renderer's xterm at the
phone's 1000-row limit, and the hidden-output reveal repaints the pane
from that copy, so a restored pane with more history lost all but its
last 1000 rows on first reveal.

src/main/runtime/headless-hydration-desktop-depth.test.ts

@@ -0,0 +1,31 @@
+import { expect, it, vi } from 'vitest'
+import { createHydrationRuntime, PTY_ID, SIZE } from './headless-hydration-ownership-test-fixture'
+
+const LINES = Array.from({ length: 1500 }, (_, i) => `L${String(i + 1).padStart(4, '0')} line`)
+
+it('hydrates main from the renderer at desktop depth so a hidden-output reveal keeps history', async () => {
+  const runtime = createHydrationRuntime()
+  const serialize = vi.fn(async (_ptyId: string, opts?: { scrollbackRows?: number }) => ({
+    data: LINES.slice(-((opts?.scrollbackRows ?? 0) + SIZE.rows)).join('\r\n'),
+    ...SIZE
+  }))
+  runtime.setPtyController({
+    write: () => true,
+    kill: () => true,
+    getForegroundProcess: async () => null,
+    getSize: () => SIZE,
+    hasRendererSerializer: () => true,
+    serializeBuffer: serialize
+  })
+
+  runtime.onPtyData(PTY_ID, '\r\n$ ', 1)
+  await runtime.model().writeChain
+
+  expect(serialize).toHaveBeenCalledWith(PTY_ID, { scrollbackRows: 5000 })
+  const snapshot = await runtime.serializeHiddenOutputRecoveryBuffer(PTY_ID, {
+    scrollbackRows: 5000
+  })
+  const restored = `${snapshot?.scrollbackAnsi ?? ''}${snapshot?.data ?? ''}`
+  expect(restored.match(/L\d{4} line/g)).toHaveLength(1500)
+  expect(restored).toContain('L0001 line')
+})

src/main/runtime/scrollback-limits.ts

@@ -1,3 +1,3 @@
-// Bound desktop xterm hydration for phones: replaying the full 50k-row buffer across IPC/headless emulation is expensive (docs/mobile-prefer-renderer-scrollback.md).
+// Bound phone snapshots: replaying the full 50k-row buffer across IPC/headless emulation is expensive (docs/mobile-prefer-renderer-scrollback.md).
 export const MOBILE_SUBSCRIBE_SCROLLBACK_ROWS = 1000
 export const MOBILE_SNAPSHOT_BYTE_BUDGET = 512 * 1024

src/main/runtime/terminal/output/headless-terminal-hydration.ts

@@ -3,7 +3,7 @@ import { splitFreebuffScreenUpdates } from '../../../../shared/freebuff-screen-s
 import { OrcaRuntimeWithSerializeMainTerminalBuffer } from './terminal-buffer-operations'
 import { observeFreebuffTerminalStatus } from '../../freebuff-terminal-status'
 import { observeCodexCapacityTerminalStatus } from '../../codex-capacity-terminal-status'
-import { MOBILE_SUBSCRIBE_SCROLLBACK_ROWS } from '../../scrollback-limits'
+import { DESKTOP_TERMINAL_SCROLLBACK_ROWS_DEFAULT } from '../../../../shared/terminal-scrollback-policy'
 import {
   detectAgentStatusFromTitle,
   normalizeTerminalTitle
@@ -57,8 +57,9 @@ export abstract class OrcaRuntimeWithMaybeHydrateHeadlessFromRenderer extends Or
       }
       try {
         // Retain normal-buffer scrollback while a TUI is in alt-screen so pre-TUI history remains restorable (#6106).
+        // Desktop depth (this emulator's retention): a hidden-output reveal repaints the pane from this model.
         const rendered = await controller.serializeBuffer!(ptyId, {
-          scrollbackRows: MOBILE_SUBSCRIBE_SCROLLBACK_ROWS
+          scrollbackRows: DESKTOP_TERMINAL_SCROLLBACK_ROWS_DEFAULT
         })
         if (
           this.headlessTerminals.get(ptyId) !== state ||