[Bug] Long assistant messages not fully scrollable in TUI (regression in 2.1.218)

Bug Description

# Bug: long assistant messages not fully scrollable in TUI (regression between 2.1.199 and 2.1.218) ## Summary On recent Claude Code builds, a single assistant message taller than the terminal window can no longer be scrolled back to its beginning. Scrolling up reveals only the last ~2–3 pages of the message, then jumps straight to pre-launch terminal scrollback (e.g. earlier shell commands). The beginning/middle of the message is unreachable in the live view. An older build does not exhibit this. ## Environment - OS: macOS (Darwin 24.6.0) - Terminal: iTerm2 3.6.9 - Install: native installer (~/.local/bin/claude → ~/.local/share/claude/versions/<ver>) - iTerm scrollback: Unlimited Scrollback = 1 (so terminal scrollback is NOT the limiter) - TERM: xterm-256color ## Repro 1. In Claude Code, produce an assistant message several screens tall (e.g. a large multi-section answer with tables). 2. After it finishes, scroll up in the terminal. Expected: scroll back through the entire message. Actual: only the last ~2–3 pages are reachable; above that the view falls through to the main-buffer scrollback from before Claude Code launched. The earlier portion of the message is gone from the live view. (Full text is still intact in the on-disk transcript at ~/.claude/projects/<slug>/<session>.jsonl, so it's a live-view/buffer issue, not lost output.) ## Version bisection (same machine, same terminal, same repro) - 2.1.199 — GOOD (full message scrollable) - 2.1.212 — GOOD (full message scrollable) - 2.1.218 — BAD (only last ~2–3 pages reachable) Regression is isolated to 2.1.218. 2.1.212 is the last-known-good build; the scrollback regression was introduced in the 2.1.213–2.1.218 range and is present in 2.1.218. ## Notes - Reproduces with iTerm "Unlimited Scrollback" enabled, so it is independent of the terminal's scrollback-lines setting. - Likely related to the app's in-TUI scroll buffer size or how tall messages are repainted in the alternate screen buffer, changed in a build after 2.1.199.

Environment Info

• Platform: darwin
• Terminal: iTerm.app
• Version: 2.1.212
• Feedback ID: de587c47-cd09-4b66-9e92-91b1fc7b3be5

Errors

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