Empty lines appear between each line in tmux scrollback/copy-mode

Description

When using Claude CLI inside tmux, empty lines appear between each line of output when scrolling back or entering copy mode. The issue only affects Claude CLI output - other command output in the same tmux session displays correctly.

Environment

• Claude CLI version: 2.1.23
• tmux version: 3.6a
• macOS version: 26.2 (Build 25C56)
• Terminals tested: Ghostty, Apple Terminal

Steps to Reproduce

1. Start a tmux session
2. Run claude to start Claude CLI
3. Interact with Claude (ask a question, get a response)
4. Exit Claude CLI (/exit or Ctrl+C)
5. Scroll up in tmux (using mouse or prefix + [)
6. Observe empty lines between each line of Claude CLI output

Expected Behavior

Output should display normally without extra blank lines, same as when running Claude CLI outside tmux.

Actual Behavior

Empty/blank lines appear between every line of Claude CLI output in tmux scrollback. The gaps:

• Appear only for Claude CLI output, not other commands
• Persist in scrollback after Claude CLI exits
• Are visible both during active Claude session and after exiting

Additional Context

• Works correctly when running Claude CLI directly in terminal (without tmux)
• Issue occurs in multiple terminals (Ghostty, Apple Terminal) - not terminal-specific
• Issue is specific to tmux + Claude CLI interaction