refactor(terminal): drop the scrollback flag; the wipe was never justified here

The previous commit added a host flag so the client could tell whether a
recovery payload carried scrollback. It cannot: every payload reaching that
branch is screen-only by construction. The ack-recovery emit serializes with
scrollbackRows: 0 literally, and the untagged resync request carries no
scrollback either. A payload that does carry scrollback arrives tagged, and the
request branch never clears.

So the flag was always false where it mattered. Delete it -- the wire field, the
host helper, the parse, and the conditional -- and simply stop emitting the
scrollback erase on this path. No wire change at all now.

src/main/runtime/rpc/methods/terminal.ts

@@ -101,17 +101,6 @@ type SnapshotFrameOptions = {
   pendingEscapeTailAnsi?: string
   /** Effective kitty flags proven at this frame's own `seq`. */
   kittyKeyboardFlags?: number
-  /**
-   * Whether `data` carries scrollback as well as the visible screen. Desktop
-   * snapshots are serialized with `scrollbackRows: 0`, so a client that erases
-   * its own scrollback to apply one destroys history the frame cannot replace.
-   */
-  scrollbackIncluded?: boolean
-}
-
-/** Whether a serialized snapshot's payload carries scrollback, not just the visible screen. */
-function snapshotCarriesScrollback(snapshot: SerializedSnapshot): boolean {
-  return Boolean(snapshot && (snapshot.scrollbackRows > 0 || snapshot.scrollbackAnsi))
 }
 
 type SerializedSnapshot = {
@@ -671,9 +660,6 @@ function sendSnapshotFrames(
         ...(typeof options.seq === 'number' && options.kittyKeyboardFlags !== undefined
           ? { kittyKeyboardFlags: options.kittyKeyboardFlags }
           : {}),
-        // Why additive on the same rule: absence means "unknown", and a client
-        // that cannot read it keeps its existing behavior.
-        ...(options.scrollbackIncluded === true ? { scrollbackIncluded: true } : {}),
         truncated: options.truncated === true,
         truncatedByByteBudget: options.truncatedByByteBudget === true
       })
@@ -1868,7 +1854,6 @@ export const TERMINAL_METHODS: RpcAnyMethod[] = [
               source: serialized.source,
               kittyKeyboardFlags: serialized.kittyKeyboardFlags,
               truncatedByByteBudget: serialized.truncatedByByteBudget,
-              scrollbackIncluded: snapshotCarriesScrollback(serialized),
               data: serialized.data
             }
           )
@@ -2362,7 +2347,6 @@ export const TERMINAL_METHODS: RpcAnyMethod[] = [
             truncatedByByteBudget: serialized?.truncatedByByteBudget,
             // Why: no serializer answered, which is not proof the pane is empty — say so instead of passing off '' as the buffer.
             unavailable: serialized ? undefined : 'no-serializable-buffer',
-            scrollbackIncluded: snapshotCarriesScrollback(serialized),
             data: serialized?.data ?? ''
           })
         } catch (error) {

src/renderer/src/runtime/remote-runtime-terminal-frame-drop-resync.test.ts

@@ -16,9 +16,7 @@ import { replaceRuntimeEnvironmentRevisions } from './runtime-environment-revisi
 
 // Why screen-only: a resync reply is serialized with `scrollbackRows: 0`, so it
 // carries the visible screen and nothing else. Erasing the client's scrollback
-// (`\x1b[3J`) to apply it would destroy history the frame cannot put back. The
-// host sets `scrollbackIncluded` when a payload does carry scrollback, and only
-// then is the full clear correct.
+// (`\x1b[3J`) to apply it would destroy history the frame cannot put back.
 const RECOVERY_SCREEN_CLEAR = '\x1b[2J\x1b[H'
 
 // Why: reproduces the silent frame-drop corruption. The server multiplex path

src/renderer/src/runtime/remote-runtime-terminal-multiplexer.ts

@@ -217,10 +217,6 @@ type RemoteRuntimeSnapshotInfo = {
   // must write it AFTER the replay reset so the next live chunk completes it
   // instead of rendering literally (#7329).
   pendingEscapeTailAnsi?: string
-  // Why: whether the payload carries scrollback as well as the screen. Absent
-  // from older hosts, which is why the recovery path treats false as "do not
-  // erase scrollback" rather than "there is none".
-  scrollbackIncluded?: boolean
 }
 
 type RemoteRuntimeSnapshotRequest = {
@@ -908,19 +904,19 @@ class RemoteRuntimeTerminalMultiplexer {
             kittyKeyboardFlags: info?.kittyKeyboardFlags
           })
         } else if (target === 'recovery') {
-          // Why: a recovery repaint must never destroy more than it replaces.
-          // `\x1b[3J` erases xterm's scrollback, but desktop snapshots are
-          // serialized with `scrollbackRows: 0`, so the frame cannot put that
-          // history back — clear the screen alone unless the host says this
-          // payload carries scrollback. An absent flag means an older host,
-          // which never sends scrollback on this path either.
-          const clearAnsi = info?.scrollbackIncluded ? '\x1b[2J\x1b[3J\x1b[H' : '\x1b[2J\x1b[H'
+          // Why the screen only, never `\x1b[3J`: that erases xterm's scrollback,
+          // and every payload reaching this branch is screen-only by
+          // construction — the ack-recovery emit serializes with
+          // `scrollbackRows: 0`, and the untagged resync request carries no
+          // scrollback either. Erasing history the frame cannot put back is
+          // silent data loss. A payload that DOES carry scrollback arrives
+          // tagged, and the request branch above never clears.
           // Why not an empty repaint: with no payload there is nothing to put
           // on screen, so applying one only blanks a pane the host could not
           // describe. A mid-escape tail still has to replay so the next live
           // chunk completes it instead of rendering literally (#7329).
           if (data) {
-            stream.callbacks.onSnapshot(`${clearAnsi}${data}`, {
+            stream.callbacks.onSnapshot(`\x1b[2J\x1b[H${data}`, {
               pendingEscapeTailAnsi: info?.pendingEscapeTailAnsi,
               seq: info?.seq,
               kittyKeyboardFlags: info?.kittyKeyboardFlags
@@ -1565,7 +1561,6 @@ function decodeSnapshotInfo(
     requestId?: unknown
     truncated?: unknown
     unavailable?: unknown
-    scrollbackIncluded?: unknown
     pendingEscapeTailAnsi?: unknown
     kittyKeyboardFlags?: unknown
   }>(payload)
@@ -1582,7 +1577,6 @@ function decodeSnapshotInfo(
     requestId: typeof raw.requestId === 'number' ? raw.requestId : undefined,
     truncated: raw.truncated === true,
     unavailable: parseTerminalSnapshotUnavailableReason(raw.unavailable),
-    scrollbackIncluded: raw.scrollbackIncluded === true,
     pendingEscapeTailAnsi:
       typeof raw.pendingEscapeTailAnsi === 'string' ? raw.pendingEscapeTailAnsi : undefined
   }

src/renderer/src/runtime/runtime-terminal-stream.test.ts

@@ -595,23 +595,6 @@ describe('remote runtime terminal multiplex ACK gate', () => {
     })
     expect(onSubscribed).toHaveBeenCalledTimes(1)
 
-    // Why scrollback may be erased here: the host states the payload carries
-    // it, so the frame replaces the history it removes.
-    injectSnapshot(
-      {
-        kind: 'scrollback',
-        cols: 120,
-        rows: 40,
-        reason: 'ack-pending-overflow',
-        truncated: false,
-        scrollbackIncluded: true
-      },
-      'restored with history'
-    )
-    expect(onSnapshot).toHaveBeenCalledWith(`\x1b[2J\x1b[3J\x1b[H${'restored with history'}`, {
-      pendingEscapeTailAnsi: undefined
-    })
-
     // Why nothing at all: this replaced the previous rule, which read an empty
     // payload as an authoritative blank and cleared on it. An empty payload is
     // the host failing to describe the pane, not proof the pane is empty, and