Tmux scrollback broken after May 26 update (v2.1.152)

Description

After the update to v2.1.152 (installed May 26, 2026), tmux scrollback no longer works in the Claude Code pane. Sessions opened before the update scrolled fine; all sessions opened after the update cannot scroll back through conversation history.

Steps to Reproduce

1. Open a new tmux pane
2. Run claude
3. Have a conversation long enough to scroll
4. Attempt to scroll up with tmux copy mode (prefix + [)

Expected Behavior

Able to scroll back through previous conversation output in the tmux pane, as was possible before v2.1.152.

Actual Behavior

Cannot scroll back beyond the current screen. Other tmux panes (not running Claude Code) scroll back normally, so the tmux scrollback limit is not the issue.

Environment

• Claude Code version: 2.1.152
• OS: macOS (Darwin 25.5.0)
• Terminal multiplexer: tmux

Hypothesis

Claude Code may have switched to using the alternate screen buffer in this update, which prevents tmux from capturing output in the main scrollback buffer. Previous versions did not appear to use the alternate screen.