[Bug] Line duplication and content loss on scroll up

**Bug Description**
just above the top of the screen, if I scroll up a few lines, it duplicates a line and loses some lines

**Environment Info**
- Platform: darwin
- Terminal: ghostty
- Version: 2.1.263
- Feedback ID: e4297a16-d2a6-492a-b9af-b895400461fa

**Errors**
```json
[]
```

No source file or test is identified in the issue. First reproduce the problem on darwin using Ghostty and Claude Code 2.1.263 while scrolling upward near the top of the screen, then trace the terminal rendering path. Done means upward scrolling no longer duplicates a line or loses content.

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