[BUG] Terminal focus events (^[[I) display as raw text on Windows

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

When Claude Code starts in Windows Terminal, the ANSI escape sequence ^[[I (focus gained event) is displayed as raw text in front of the header instead of being consumed.

What appears:

^[[I
 ▐▛███▜▌   Claude Code v2.1.7
▝▜█████▛▘  Opus 4.5 · Claude Max
  ▘▘ ▝▝    ~\source\repos\...

Expected behavior:
The focus event escape sequence should be consumed/filtered, not displayed as visible text.

Root cause:
Windows Terminal sends focus reporting events (ESC[I for focus gained, ESC[O for focus lost) when the terminal window gains/loses focus. These are DECSET 1004 focus events. The TUI is not consuming these escape sequences from stdin.

Related issue:
This is similar to #17787 (macOS cursor position responses leaking) - both involve ANSI escape sequences not being properly consumed by the TUI.

What Should Happen?

The Claude Code TUI should consume/filter terminal focus events (DECSET 1004 sequences) so they don't appear as visible text. The header should display cleanly without any ^[[I or ^[[O escape sequences showing.

Error Messages/Logs

Steps to Reproduce

1. Open Windows Terminal (or another terminal with focus reporting enabled)
2. Run claude to start Claude Code
3. Click away from the terminal window, then click back to give it focus
4. Observe that ^[[I appears in front of the header when the terminal gains focus

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.7

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

_No response_