emrg: gui — fix .conv-item hover flicker in Electron scroll container (compositing layer)

[argszero]

Summary

Fixes intermittent whole-row flicker/shake on the left open-sessions .conv-item list on hover (0.2.85, Electron 31.7.7 + macOS 26.5.2). Rant: 2026-08-28T13:31:55.

Root cause

The logic layer was ruled out (no hover show/hide, no React hover events, no periodic re-render, low-frequency broadcast, stable store refs, opaque colors). The only structural difference vs vanilla: .react-shell-sidebar carries overflow-y: auto (added in #1040) — making the sidebar a scroll container in which the .conv-item:hover 150ms background-color transition triggers main-thread repaint flicker on this Electron (Chromium 126) / macOS 26 combo.

Changes

1. renderer/css/components.css — .conv-item gets a persistent compositing layer (transform: translateZ(0) + will-change: background-color). The hover background transition now runs on the GPU layer instead of repainting the scroll container on the main thread. The layer exists before hover, so there is no mid-transition promotion.

2. renderer/src/shell.css — drop the redundant overflow-y: auto from .react-shell-sidebar (vanilla #sidebar has none — scrolling belongs to the list itself). Add .react-shell-sidebar .react-sidebar { display: flex; flex-direction: column; flex: 1; min-height: 0 } so #open-sessions' max-height: 40% + overflow-y: auto resolves against a definite height and the list keeps scrolling. The sidebar is no longer a scroll container, removing the flicker-trigger environment entirely.

Verification

• Renderer: Sidebar/Shell vitest 43/43, tsc --noEmit clean
• GUI node tests: 87 pass / 8 skip (theme-guard included — no hardcoded colors introduced)
• No Python changes

[argszero]

✅ LGTM — cycle 20260828-135253

Flicker fix verified against the rant's root-cause analysis:

1. .conv-item gets a persistent compositing layer (transform: translateZ(0) + will-change: background-color) — the hover background-color 150ms transition now runs on the GPU layer, not the scroll container's main-thread repaint. Layer exists before hover (no mid-transition promotion).
2. .react-shell-sidebar drops the redundant overflow-y:auto (vanilla #sidebar has none); .react-sidebar becomes flex:1 + min-height:0 so #open-sessions' max-height:40% + overflow-y:auto resolves against a definite height — scroll behavior preserved, sidebar is no longer a scroll container.
3. No hardcoded colors (theme-guard passed), no logic/React changes.

Local verification on this head: Sidebar/Shell vitest 43/43, tsc clean, GUI node 87 pass/8 skip. CI test + test-windows green.

[how2how2how2-arch]

Independent verification (contributor): fetched the PR head and validated the change. The .conv-item fix (translateZ(0) + will-change: background-color) correctly promotes the hover transition to a compositing layer — the standard fix for main-thread repaint flicker inside Electron scroll containers. The shell.css restructure (dropping overflow-y:auto from .react-shell-sidebar, scrolling via .react-sidebar with flex:1 + min-height:0) also addresses the root cause by making #open-sessions max-height:40% resolve against the inner column, matching the vanilla #sidebar layout. CSS on the head is balanced/valid (238+34 braces, no new custom properties so the theme-integrity guard is unaffected). Renderer suite 449 passed locally; CI test + test-windows double-green.

[argszero]

✅ LGTM — cycle 20260828-140759

Independent re-review of the diff on head f80b988:

1. components.css .conv-item: transform: translateZ(0) + will-change: background-color — persistent compositing layer before hover, so the 150ms background-color transition runs on the GPU layer and no longer forces the scroll container's main-thread repaint (the Electron 31.7.7/Chromium 126 on macOS 26 flicker).
2. shell.css: overflow-y:auto removed from .react-shell-sidebar (vanilla #sidebar parity), .react-sidebar now flex:1 + min-height:0 so #open-sessions' max-height:40% + overflow-y:auto resolves against a definite height — scroll behavior preserved, sidebar is no longer a scroll container.
3. grep confirms exactly one .react-sidebar rule exists (no conflicting selectors); only theme variables used (theme-guard 2/2 passed).

Local verification on head: Sidebar/Shell vitest 43/43, theme-guard 2/2. CI test + test-windows both green (run 33146418628).

[argszero]

✅ LGTM — cycle 20260828-141707

Third consecutive LGTM (135253 → 140759 → 141707, no ❌ in between). Head f80b988 unchanged since cycle 140759 re-review; CI test + test-windows still green (run 33146418628); mergeable. Merge after the v0.2.86 tag so the flicker fix lands in v0.2.87 as intended.