[BUG] tmux copy mode displays extra blank lines between each line (visual only, v2.1+)

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 using Claude Code inside tmux and entering copy mode to scroll back through the
output, every line appears with an extra blank line between it. This is a
visual-only issue - when selecting and copying the text, the pasted content does
not contain these blank lines.

What Should Happen?

## Expected Behavior

Lines should display normally in copy mode, matching how they appear during live
output.

## Actual Behavior

• Visual: Double line spacing (blank line between each line)
• Copy/Paste: Normal (no extra blank lines when pasted)

## Additional Context

This suggests the issue is related to ANSI escape sequences or cursor positioning
commands that leave "ghost lines" in tmux's scrollback buffer, rather than actual
newline characters in the output stream.

The regression from 2.0 to 2.1 may be related to changes in the TUI rendering system.

Error Messages/Logs

Steps to Reproduce

1. Run Claude Code inside a tmux session
2. Have Claude generate some output (any conversation)
3. Enter tmux copy mode (Ctrl+b [ or your prefix)
4. Scroll up through the scrollback buffer
5. Observe: every line of Claude Code output has an extra blank line below it

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.0.15

Claude Code Version

2.1.19

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

iTerm2

Additional Information

## Version Information

• Claude Code version: 2.1.x (started occurring in 2.1, did NOT occur in 2.0)
• Current version tested: 2.1.19
• tmux version: 3.6a
• TERM: screen-256color
• OS: macOS (Darwin 25.2.0), Ubuntu Linux 24.04.3