codex - 💡(How to fix) Fix Repro artifacts for line truncation / missing middle output line (issue #15380)

Follow-up to #15380 with deterministic repro artifacts and code changes, posted as an issue instead of a PR.

Observed behavior (user-facing)

While reading assistant output in the terminal, lines can disappear from the middle of previously rendered output.

Typical pattern reported by the original author:

• Start from the latest assistant message.
• Scroll upward to earlier output.
• Notice that one or more complete lines are missing in terminal history.
• The missing lines are present in the transaction/log view, so this appears to be a terminal rendering/scrollback insertion problem rather than missing model content.

Additional report context:

• The original reporter also sees this in Windows Terminal without tmux.

What this issue adds

I prepared local changes that add:

1. repro/tmux_scrollback_repro.sh
   • Deterministic harness that emits a Codex-like stream (S-0001..S-0400) and validates sequence gaps from captured terminal history.
2. repro/line_truncation_repro.md
   • Step-by-step repro instructions and expected/observed outcomes.
3. codex-rs/tui/tests/suite/vt100_history.rs
   • Focused incremental-streaming baseline test against the in-process vt100 backend.

Repro results in this environment

Running:

./repro/tmux_scrollback_repro.sh

consistently reports one missing middle line, e.g. S-0391 (missing_count=1).

Control path:

./repro/tmux_scrollback_repro.sh --plain

reports missing_count=0.

Patch summary (local)

• Added test: codex-rs/tui/tests/suite/vt100_history.rs
  • incremental_stream_history_keeps_all_numbered_lines
• Added repro harness: repro/tmux_scrollback_repro.sh
• Added repro notes: repro/line_truncation_repro.md

If maintainers prefer, I can re-post this as a PR once permissions are available.