fix: address pr-bug-scan validated finding from #6532

For alt-screen restore, emit viewport erase \x1b[2J\x1b[H (no \x1b[3J) before replay so stale pre-hide cells are wiped while scrollback is preserved.

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

@@ -6158,12 +6158,17 @@ describe('connectPanePty', () => {
     await flushAsyncTicks(20)
 
     expect(getMainBufferSnapshot).toHaveBeenCalledWith('pty-id', { scrollbackRows: 5000 })
-    // Why: the destructive clear wipes xterm's scrollback. Alt-screen TUIs keep
-    // their history in xterm, so restoring one must NOT emit the clear.
+    // Why: \x1b[3J wipes xterm's scrollback. Alt-screen TUIs keep their history
+    // in xterm, so restoring one must NOT emit the scrollback-clearing form.
     expect(pane.terminal.write).not.toHaveBeenCalledWith(
       '\x1b[2J\x1b[3J\x1b[H',
       expect.any(Function)
     )
+    // Why: but the visible viewport must still be erased before replay.
+    // The SerializeAddon repaint is minimal (non-erasing moves for blank cells,
+    // omitted blank rows), so without \x1b[2J, cells the TUI blanked while the
+    // pane was hidden keep their stale pre-hide glyphs behind the restored frame.
+    expect(pane.terminal.write).toHaveBeenCalledWith('\x1b[2J\x1b[H', expect.any(Function))
     expect(pane.terminal.write).toHaveBeenCalledWith('altscreen-snapshot\r\n', expect.any(Function))
     disposable.dispose()
   })

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

@@ -3242,6 +3242,15 @@ export function connectPanePty(
         // clearing on restore loses scroll-up after a hidden->visible return.
         // Mirrors the attach-time guard in pty-transport.ts.
         writeReplayData('\x1b[2J\x1b[3J\x1b[H')
+      } else {
+        // Why: skip only \x1b[3J (the scrollback wipe) for alt-screen, but
+        // still erase the visible viewport with \x1b[2J before replay. The
+        // SerializeAddon repaint is minimal — it emits non-erasing cursor
+        // moves for default-background blanks and omits fully-blank rows — and
+        // \x1b[?1049h is a no-op when xterm is already on the alt buffer, so
+        // without this erase, cells the TUI blanked while hidden keep their
+        // stale pre-hide glyphs behind the restored frame.
+        writeReplayData('\x1b[2J\x1b[H')
       }
       writeReplayData(snapshot.data)
       writeReplayData(POST_REPLAY_LIVE_SNAPSHOT_RESET)