[Bug] Severe terminal scroll lag starting in 2.1.112 (statusline ruled out)

Environment

• Claude Code: 2.1.112 (regression; prior versions through ~2.1.108 were smooth in the same setup)
• OS: Linux 6.19.6 (Ubuntu noble, t2 kernel on MacBook Pro)
• Terminal: xterm-256color, tmux 3.4
• Relevant env:
• CLAUDE_CODE_NO_FLICKER=1
• CLAUDE_CODE_DISABLE_ADAPTIVE_THINKING=1
• CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1

Symptom

Scrolling the terminal buffer (mouse wheel, PgUp/PgDn, Shift+Up/Down) is severely laggy — the UI hitches for hundreds of ms per scroll event, making the CLI effectively unusable for reviewing earlier output. Appeared the day I auto-updated to 2.1.112; no other environment changes.

Reproduction

1. Start claude and run any command that produces moderately long output (e.g. a few tool calls with diffs, or a long agent response).
2. Try to scroll the buffer up with mouse wheel or PgUp.
3. Observe stutter / delayed repaint on every scroll tick.

Expected

Scrolling should be smooth as it was on 2.1.108 and earlier (same terminal, same tmux, same settings).

Diagnostic done

• Disabled statusline (removed statusLine block from ~/.claude/settings.json and set claude-hud@claude-hud: false). Restarted Claude Code.
• Lag still present with statusline fully off.
• This suggests the regression is in the core renderer / buffer handling in 2.1.111–2.1.112, not in plugin/statusline invocation frequency.

Related prior rendering issues

• #42087 — terminal rendering: 100% write ratio, input latency during streaming
• #48810 — /context output overlaps with diff/tool output
• #43571 — rendering corruption with concurrent background agents

These suggest the renderer has been churning recently; this report is specifically about scroll performance regression in 2.1.112 independent of statusline.

Requests

• Confirm whether buffer redraw / scroll handling changed in 2.1.111 or 2.1.112.
• If possible, provide a flag to fall back to the prior renderer while this is investigated.

Notes

• Statusline ruled out (still laggy without it).
• Have not yet isolated tmux vs. bare terminal; happy to retest outside tmux if it helps narrow this down.