[BUG] Agent view: scrolling inside attached session leaves stale left-column artifacts (Windows Terminal 2.1.141)

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 attached to a session inside agent view (claude agents → select a row → Enter), scrolling the transcript leaves stale character cells in the leftmost columns. The artifacts appear to be fragments of previously rendered frames (markers such as X, 2., 3., //, ⚠, 🎯 from earlier content) that the redraw fails to clear.

Ctrl+L forces a full redraw and clears the artifacts, but they reappear on the very next scroll action.

Key observation: This bug is specific to the agent-view attach path. Launching a session directly with claude and scrolling produces NO artifacts. The stale left-column cells only appear when attaching to a session via claude agents → select row → Enter.

The transcript content that reliably triggers this contains:

• CJK (Traditional Chinese) text
• Box-drawing characters forming tables (├─┼─┤, │, etc.)
• Emoji status markers (✅ ❌ ⚠ 🎯)
• Mixed-width characters on the same line

Reproduction

1. Have a session whose transcript contains CJK + a box-drawing table + emoji markers.
2. Run claude agents.
3. Select the session and press Enter to attach.
4. Scroll up/down with mouse wheel or PgUp / PgDn.
5. Observe stale character cells along the leftmost columns of the viewport.
6. Press Ctrl+L — artifacts clear.
7. Scroll again — artifacts return immediately.
8. Exit agent view and launch claude directly — scroll the same content — NO artifacts.

Environment

• Claude Code version: 2.1.141
• OS: Windows 11
• Terminal: Windows Terminal (default profile, default font: Cascadia Mono)
• Shell: PowerShell
• CLAUDE_CODE_NO_FLICKER: tested with both 0 and 1 — both reproduce identically

What I've Already Ruled Out

• Not terminal-emulator specific. Reproduced identically on both Windows Terminal and WezTerm, so this is not a wide-character / column-width issue in the emulator.
• Not NO_FLICKER mode specific. Reproduces with CLAUDE_CODE_NO_FLICKER=0 and =1, suggesting the bug is in the higher-level diff/redraw layer rather than the rendering backend.
• Not fixed by recent CHANGELOG entries. Running 2.1.141, which already includes:
• "Fixed scroll behavior in Windows Terminal and VS Code when attached to background sessions"
• "Fixed border-embedded text overflowing on CJK/emoji due to visual cell width miscalculation"
• "Fixed wide markdown tables leaving a stale bordered render in terminal scrollback while streaming"

These look related but do not cover this specific symptom.

<img width="1280" height="758" alt="Image" src="https://github.com/user-attachments/assets/9f745e02-6f06-40ea-a345-30c6ef0674c5" />

What Should Happen?

Scrolling the transcript should fully repaint every column on every scroll tick automatically, matching what Ctrl+L produces.

Error Messages/Logs

### Suspected Root Cause

The direct `claude` launch path does not reproduce this, which strongly suggests the bug is in the **attach/re-attach viewport initialization logic** — the code path that restores an existing session's transcript into the TUI — not in the general scroll rendering layer. Likely the incremental diff is failing to track column width correctly for mixed CJK / box-drawing / emoji content when the viewport is restored from a detached state.

### Screenshots

（請將我提供的兩張截圖 attach 在此處）

Steps to Reproduce

1. Have a session whose transcript contains CJK + a box-drawing table + emoji markers.
2. Run claude agents.
3. Select the session and press Enter to attach.
4. Scroll up/down with mouse wheel or PgUp / PgDn.
5. Observe stale character cells along the leftmost columns of the viewport.
6. Press Ctrl+L — artifacts clear.
7. Scroll again — artifacts return immediately.
8. Exit agent view and launch claude directly — scroll the same content — NO artifacts.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

Windows 11

Claude Code Version

2.1.141

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

_No response_