Scroll stutter regression in v2.1.216 (fullscreen mode) — v2.1.215 smooth in identical environment

Summary

Mouse-wheel scrolling through conversation history in fullscreen rendering mode stutters noticeably on v2.1.216. Running the v2.1.215 binary in an identical environment (same terminal, same settings, same conversation length) scrolls smoothly. This is a clean A/B-confirmed regression in the 2.1.215 → 2.1.216 window.

Environment

• Claude Code: 2.1.216 (native installer, inside WSL2 Ubuntu)
• Windows Terminal 1.24.11911.0 — hardware D3D11 rendering confirmed active (d3d11.dll/dxgi.dll loaded in WindowsTerminal.exe, no d3d10warp.dll, i.e. not a WARP/software fallback)
• WSL2: kernel 6.18.33.2-microsoft-standard-WSL2, WSL 2.7.10.0, Windows 10 build 19045
• Hardware: Ryzen 9 5950X (28 vCPUs assigned to WSL), 54 GB RAM available, GTX 1060 (driver 560.94), local session (no RDP)
• Fullscreen mode enabled via CLAUDE_CODE_NO_FLICKER=1 (also "tui": "fullscreen" in settings)
• Terminal grid: 39 rows × 91 cols

Steps to reproduce

1. On 2.1.216, open a conversation a few pages long in fullscreen rendering mode.
2. Scroll through it with the mouse wheel → visible stutter/jank.
3. Run the previous binary directly (~/.local/share/claude/versions/2.1.215), same conversation length, same environment → smooth scrolling.

Isolation evidence (environment ruled out)

• Pure Windows Terminal scrollback scrolling (seq 1 100000, then wheel through the scrollback) — smooth → terminal-side rendering is fine.
• less --mouse over a 100k-line file — exercising the same path as fullscreen mode (alt screen + mouse-wheel events + WSL → ConPTY → Windows Terminal) — smooth → the ConPTY pipe is fine.
• Instantaneous CPU of the claude process while scrolling (1-second /proc/<pid>/stat deltas): peaks at ~41% of a single core → not CPU-bound.
• GPU hardware rendering verified as above; no software fallback, no RDP.

Regression window

2.1.215 → 2.1.216 (auto-updated 2026-07-21; the regression became visible on the first fresh launch after a reboot).

Workaround

Pin the previous version and disable auto-update:

ln -sfn ~/.local/share/claude/versions/2.1.215 ~/.local/bin/claude
export DISABLE_AUTOUPDATER=1