[Desktop] Session panel width animation costs ~0.24s of main-thread work per toggle #48701

Description

Opening/closing the review panel, terminal or file tree animates the width of the chat panel and the side panel over 240ms (packages/app/src/pages/session.tsx and packages/app/src/pages/session/session-side-panel.tsx).

Measured in a real desktop instance (1.18.29, via CDP), per panel toggle:

• layout: ~64ms
• script: ~16ms
• total main-thread task time: ~239ms

I also tried contain: layout paint and contain: layout on the panel: no measurable gain (the cost is inside the panel — content reflow plus the virtualized timeline re-measuring rows each frame).

Shortening the animation to 160ms cuts the work by ~25% (task 239ms → 181ms, layout 64ms → 45ms) while keeping the slide. A no-animation control comes in at ~32ms, but that changes the feel, so it is not proposed.

Plugins

none

OpenCode version

OpenCode Desktop 1.18.29

Steps to reproduce

1. Open a session with content
2. Toggle the review panel (or terminal/file tree)
3. Profile with DevTools Performance: layout runs every frame during the 240ms animation

Screenshot and/or share link

No screenshot — no visual difference at rest.

Operating System

CachyOS (Arch Linux), KDE Plasma 6.7.5, Wayland

Terminal

n/a (desktop app)