Double Esc backtrack causes stack overflow

Summary

• double-pressing Esc in the main chat view (with an empty composer) triggers the backtrack preview
• subsequent Esc events recurse through open_backtrack_preview → step_backtrack_and_highlight without advancing selection when there are no user messages to highlight
• the loop never terminates, and the process crashes with hread 'main' has overflowed its stack

Reproduction Steps

1. Start the TUI in a project with an existing conversation history (e.g., run codex resume --dangerously-bypass-approvals-and-sandbox)
2. Ensure the composer is empty, then press Esc twice in quick succession
3. Observe the stack overflow crash (STATUS_STACK_OVERFLOW on Windows)

Expected Behavior

• the transcript overlay should highlight the latest user message or stop at the beginning of history without crashing

Actual Behavior

• Esc triggers an infinite loop in backtrack preview, causing a stack overflow

Additional Context

• Windows 11 (build 26100), PowerShell terminal
• codex commit: b0e4fc29 (fix/paste-burst-newline branch)
• running codex.exe resume --dangerously-bypass-approvals-and-sandbox
• no prompt text in the composer; crash occurs even without interacting further.