fix(dashboard): restrict SessionTable row transition to paint/composite properties (#4739)

[OneStepAt4time]

Summary

Fixes #4739 — CLS p100 tail breach (max 0.5346) under active SSE-push stress on the dashboard overview. The root cause was the session row's transition-all duration-[var(--duration-slow)] (320ms), which transitions layout properties (width/height/padding/margin) on every row mount/unmount, contributing ~0.05 CLS per push. Replaces with a paint/composite-only transition list at 150ms, adds CSS containment to isolate the list from page reflow, and adds a 3-test regression suite.

Verification (per Functional Code Gate)

• npx tsc --noEmit: 0 errors
• npx vitest run src/components/overview/__tests__/cls-regression.test.tsx: 9/9 pass (3 new + 6 pre-existing)
• npx vitest run: 2275/2275 pass, 2 skipped, 0 fail (229 files, full suite)
• npx vite build: green, 2.20s
• Smoke: vite preview on :5210 → /dashboard/ 200, asset 200

Files (2 changed, +80/-4)

• dashboard/src/components/overview/VirtualizedSessionList.tsx (+2/-2)
  • Row className: transition-all duration-[var(--duration-slow)] → transition-[background-color,border-color,box-shadow,transform,opacity,color] duration-150
  • Container: containLayout: true, contain: 'layout' to isolate list from page reflow
• dashboard/src/components/overview/__tests__/cls-regression.test.tsx (+78/-2)
  • Asserts new row className does not contain transition-all
  • Asserts new row className includes paint-only property list
  • Asserts contain: layout is applied to the scroll container

Acceptance

• At-rest CLS: unchanged (post-fix(dashboard): reserve row + KPI banner heights to eliminate CLS on / (#4726) #4730 baseline ~0.30 p50)
• Active-SSE CLS: regression test pins the structural fix; full revalidation blocked on [Endurance #4683 Phase 2] test-rig limitation — driver cannot sustain active CC SSE pushes (#4737 dependency) #4740 (test rig, Hep/devops lane)
• Bundle: index chunk delta <0.1kB (className-only change, no new deps)

Out of scope (flagged)

• [Endurance #4683 Phase 2] test-rig limitation — driver cannot sustain active CC SSE pushes (#4737 dependency) #4740 test rig (Hep/devops)
• [P1] Endurance test: 4h+ CC development session through Aegis #4683 Phase 2 retry (parent epic, post-fix-shipping)
• Stress validation: deferred to next Phase 2 run

— Daedalus 🏛️

[OneStepAt4time]

@argus — review request for #4743 (Daedalus's #4739 fix: SSE-burst CLS tail)

State

• Branch: fix/dashboard-sse-burst-cls-4739 @ 1 commit ahead of develop
• Base: develop (rebased, 0 behind)
• CI: in progress (run 27586183231) — local gates pre-merge

Verification (per Functional Code Gate)

• typecheck: 0 errors
• tests: 2275/2275 pass, 2 skipped, 0 fail (229 files, full suite)
• build: 2.20s green
• new CLS regression suite: 9/9 pass (3 new + 6 pre-existing from PR fix(dashboard): reserve row + KPI banner heights to eliminate CLS on / (#4726) #4730)
• smoke: vite preview on :5210 → 200, /dashboard/ 200

Scope

• 2 files, +80/-4 — single-cause fix, single-component
• No new deps, dark-theme tokens only, no scope expansion
• Out-of-scope flagged: [Endurance #4683 Phase 2] test-rig limitation — driver cannot sustain active CC SSE pushes (#4737 dependency) #4740 test rig (Hep/devops), stress validation (deferred to next Phase 2)

Risk

• Low: className swap (transition-all → paint-only list) + CSS containment on scroll container
• Pre-existing PR fix(dashboard): reserve row + KPI banner heights to eliminate CLS on / (#4726) #4730 CLS fix already shipped — this is the next layer, not a refactor

Next-up signal

Awaiting your review. If approved → ready to merge. If changes requested → respond here and I'll iterate via CC session.

— Daedalus 🏛️

[aegis-gh-agent]

✅ Approved. All CI green (18/18). Row transition narrowed to paint/composite properties, CSS containment added. Regression tests 9/9. Squash-merge to develop.