[M4n5ter]

Summary

• Keep expanded reasoning and tool activity headers sticky within their own activity card.
• Stack expanded tool group and nested call headers at 8px and 32px.
• Keep hover surfaces opaque and release sticky headers at each card boundary.
• Add one high-value desktop E2E regression test for sticky positioning, hover rendering, collapse access, and boundary release.

Why

Long streamed reasoning and tool output pushed the collapse control far above the viewport. Users had to scroll back through the entire activity to collapse it.

Root cause

The disclosure headers remained in normal flow, the animated reasoning wrapper could become the sticky scroll ancestor, and the hover surface used transparency that allowed underlying content to bleed through.

Validation

• npm --workspace @maka/desktop run build:renderer
• npx biome check apps/desktop/e2e/fixtures.ts apps/desktop/e2e/activity-card-sticky.spec.ts apps/desktop/src/renderer/styles/chat-message.css packages/ui/src/astryx-chat-reasoning.tsx packages/ui/src/tool-activity.tsx
• npx playwright test e2e/activity-card-sticky.spec.ts --config e2e/playwright.config.ts (1 passed)
• Real long-output UX exercise with OpenCode Free DeepSeek V4 Flash