New scroll UI breaks tmux copy mode workflow (scrollback no longer accessible)

Description

Since the introduction of the internal scroll viewport (visible as the "Jump to bottom (ctrl+End)" button), Claude Code output no longer goes into tmux's scrollback buffer. This breaks a common terminal workflow where users enter tmux copy mode and navigate output using vim motions.

Previous behavior

Claude Code output was written to stdout → went into tmux's scrollback buffer → users could enter copy mode (Prefix + [) and use vim motions (j/k, C-u/C-d, G, /search) to navigate and select text comfortably.

Current behavior

Claude Code now manages its own scroll viewport. Output does not appear in tmux's scrollback. Entering copy mode shows an empty or stale buffer — the Claude Code content is inaccessible via copy mode entirely. The only way to scroll is via touchpad or mouse wheel within Claude Code's own UI.

Impact

This is a regression for keyboard-driven / tmux-heavy workflows. Copy mode + vim motions is significantly more efficient than reaching for a mouse to scroll through long responses.

Suggested fix

Add a flag or config option to disable the internal scroll viewport and revert to writing output directly to stdout/scrollback:

• CLI flag: --no-scroll-ui or --no-alternate-screen
• Settings key: "scrollUI": false

Environment

• Claude Code version: 2.1.143
• OS: Linux (Arch), kernel 6.18.9
• Terminal: tmux with set -g mouse on, mode-keys vi
• Shell: zsh