[BUG] iTerm2 + tmux: mouse wheel hijacked to input history, scrollback renders broken past viewport (2.1.138 + 2.1.139)

Environment

• Claude Code: 2.1.139 (reproduces on 2.1.138 too)
• Terminal: iTerm2 on macOS 26.1
• Multiplexer: tmux 3.6a
• Install: native (~/.local/share/claude/versions/)

Summary

Scrollback regression appeared on 2.1.138 and persists on 2.1.139 inside iTerm2 + tmux:

1. Mouse wheel is hijacked — scrolling up cycles previous input history instead of the chat transcript.
2. Option+wheel moves through scrollback, but any text past the terminal viewport height renders corrupted or disappears (partial lines, missing rows, ghost characters).
3. PgUp / PgDn is the only mechanism that renders prior content correctly — one screen at a time, unworkable for copying long messages.

The same setup worked correctly before 2.1.138.

Steps to reproduce

1. macOS + iTerm2 + tmux 3.6a
2. Run claude (2.1.138 or 2.1.139)
3. Generate a multi-screen response
4. Mouse wheel → input history cycles instead of scrolling chat
5. Option+wheel → scrollback moves but renders corruption past the viewport
6. PgUp → only working method, one viewport at a time

What I've tried

• claude update (2.1.138 → 2.1.139): no improvement
• 2.1.123 (cached locally) does not exhibit this

Impact

Reading and copying long outputs is the core read-loop of the tool. PgUp-only scrollback is significantly degrading day-to-day use; planning to downgrade to 2.1.123 as a workaround.

Possibly related

• #56546 Fullscreen TUI janky/non-smooth scrolling
• #56881 missing docs for CLAUDE_CODE_DISABLE_ALTERNATE_SCREEN
• #51393 scroll regression in Windows Terminal since 2.1.x
• #55826 [Meta] Scrollback after compaction

May be a manifestation of alternate-screen / fullscreen TUI mode interacting badly with tmux's mouse capture — the iTerm2 + tmux combination seems to be the trigger.