[BUG] Display/output corruption with Claude 2.1.x in GNU screen (lack of synchronized output)

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?

After upgrading from Claude Code 2.0.76 to 2.1.12, the output is corrupted:

<img width="1634" height="1228" alt="Image" src="https://github.com/user-attachments/assets/297061bb-6bb8-43b0-b063-184c0b9d284f" />

<img width="1634" height="1228" alt="Image" src="https://github.com/user-attachments/assets/3b703bb0-3905-494e-9b5a-5c8bb698971f" />

What Should Happen?

No corruption.

Error Messages/Logs

Steps to Reproduce

I had Claude analyze the issue and here’s what it concluded:

## Root Cause Analysis

The issue appears to be related to synchronized output (DECSET 2026), a terminal feature for flicker-free rendering:

### tmux supports it (input.c):
``c
case 2026: /* synchronized output */
screen_write_start_sync(ictx->wp); // CSI ?2026h
...
case 2026: /* synchronized output */
screen_write_stop_sync(ictx->wp); // CSI ?2026l
``

### GNU screen does NOT support it (ansi.c):
Screen's DECSET handler has cases for modes 1, 2, 3, 5, 6, 7, 9, 25, 47, 1047-1049, 1000-1006, 2004 — but no case for 2026. Unknown modes are silently ignored.

### Why this causes corruption:

1. Synchronized output wraps rapid updates in \e[?2026h ... \e[?2026l so terminals buffer and render atomically
2. In tmux: output is buffered until sync ends, then rendered all at once
3. In screen: escape sequences are ignored, output flows unbuffered
4. The application's cursor positioning assumptions depend on atomic rendering
5. When those assumptions fail, cursor position desynchronizes, causing display corruption

### Evidence of change:

Comparing the bundled cli.js:

| Metric | v2.0.76 | v2.1.12 |
|--------|---------|---------|
| TMUX env references | 1 | 18 |
| process.env.TMUX checks | 0 | 3 |
| isTerm checks | 4 | 12 |

The newer version has significantly more terminal multiplexer detection, suggesting it enables tmux-specific features like synchronized output.

## Expected Behavior

Claude Code should detect when running in GNU screen (via $STY environment variable) and disable synchronized output or other tmux-specific features that screen doesn't support.

## Workaround

Pin to claude-code 2.0.76 or use tmux instead of screen.

## Suggested Fix

Check for process.env.STY (set by screen) and disable synchronized output when detected, similar to how the code already checks for process.env.TMUX."

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.0.76

Claude Code Version

2.1.12

Platform

Anthropic API

Operating System

Other Linux

Terminal/Shell

Other

Additional Information

_No response_