feat(issue-detail): add jump controls

[get6]

What does this PR do?

Adds a one-click jump control to the issue detail page for long activity/comment threads. The control keeps the issue detail page document-shaped instead of changing react-virtuoso follow behavior: it only appears when the page is actually scrollable, shows a single contextual direction, and stays secondary to the global Chat FAB. Desktop and mobile demo videos are included below to show the final placement and interaction.

Thinking path: issue detail currently has a custom scrollContainerEl for the timeline/document area, while Chat FAB already owns the primary bottom-right action. A small contextual control above that FAB solves the long-thread navigation pain without introducing competing primary actions or auto-scroll behavior that would make issue detail feel like chat.

Related Issue

Closes #2726

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Refactor / code improvement (no behavior change)
• Tests (adding or improving test coverage)
• Documentation update
• CI / infrastructure

Changes Made

• packages/views/issues/components/issue-detail.tsx
  • Adds scrollability / near-bottom tracking for the issue detail scroll container.
  • Renders one floating jump button for long threads only.
  • Shows ↓ / “Jump to latest activity” when not near the bottom.
  • Shows ↑ / “Jump to top” when near the bottom.
  • Hides the control while the chat window is open.
• packages/views/issues/components/issue-detail.test.tsx
  • Adds coverage for the contextual jump direction and scroll target.
  • Adds coverage for hiding the control when chat is open.
• packages/views/locales/en/issues.json
  • Adds English labels for the jump control.
• packages/views/locales/zh-Hans/issues.json
  • Adds zh-Hans labels for the jump control.

How to Test

1. Run pnpm --filter @multica/views exec vitest run issues/components/issue-detail.test.tsx.
2. Run pnpm --filter @multica/views typecheck.
3. Run pnpm --filter @multica/views lint.
4. Manual UI check: open a long issue thread, verify the button appears above the Chat FAB, jumps down from the top/middle, switches to jump up near the bottom, and disappears when the chat window is open. See the desktop/mobile videos below for the expected interaction.

Checklist

• I have included a thinking path that traces from project context to this change
• I have run tests locally and they pass
• I have added or updated tests where applicable
• If this change affects the UI, I have included before/after screenshots/videos
• I have updated relevant documentation to reflect my changes
• If I added a new runtime / coding tool / UI tab, I synced the change to landing copy (apps/web/features/landing/i18n/) and relevant docs (apps/docs/content/docs/)
• If this PR touches Chinese product copy, I checked it against apps/docs/content/docs/developers/conventions.zh.mdx (terminology, mixed-rule for task / issue / skill)
• I have considered and documented any risks above
• I will address all reviewer comments before requesting merge

Risk: low-to-moderate UI risk. The control is scoped to issue detail, uses the existing scroll container, and avoids changing virtualized timeline follow behavior. Main risk is visual overlap on small screens; it is positioned above the existing Chat FAB and hidden when chat is open. The attached desktop/mobile videos cover the expected placement and interaction.

Docs: no product docs update needed; this is a small issue-detail affordance rather than a documented feature surface.

Screenshots/videos: desktop and mobile demos are included below.

AI Disclosure

AI tool used: Multica Agent / Hermes

Prompt / approach:
Implemented from the linked feature issue and live product context: preserve issue detail as a document-shaped page, avoid Chat FAB conflict, add one contextual jump control, and validate with targeted unit tests plus typecheck/lint.

Screenshots (optional)

Desktop

Download desktop video

Mobile

Download mobile video

[vercel]

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[Copilot]

Pull request overview

Adds a floating “jump” control to the Issue Detail timeline to help users quickly navigate long activity threads, including i18n strings and regression tests.

Changes:

• Introduces a single floating jump button that toggles between “jump to latest activity” and “jump to top” based on scroll position, and hides while chat is open.
• Adds English and zh-Hans locale strings for the jump control labels.
• Adds regression tests covering the jump control behavior and chat-open hidden state.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
packages/views/locales/zh-Hans/issues.json	Adds zh-Hans labels for jump-to-top / jump-to-latest.
packages/views/locales/en/issues.json	Adds English labels for jump-to-top / jump-to-latest.
packages/views/issues/components/issue-detail.tsx	Implements scroll-state tracking and renders the floating jump control.
packages/views/issues/components/issue-detail.test.tsx	Adds tests validating jump control behavior and chat-open hiding.

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[get6]

Addressed the Copilot review feedback in 5ebe1e9:

• Explicitly scroll to scrollHeight - clientHeight instead of relying on browser clamping.
• Throttle scroll-state recalculation with requestAnimationFrame.
• Avoid redundant React state updates when the computed booleans have not changed.
• Updated the jump-control test expectation for the explicit max scroll target.

Validation:

• pnpm --filter @multica/views exec vitest run issues/components/issue-detail.test.tsx
• pnpm --filter @multica/views typecheck
• pnpm --filter @multica/views lint (passes with pre-existing warnings)

[get6]

Adjusted the jump control placement after local UX feedback: the floating button now sits higher above the bottom composer area instead of hugging the lower edge. Verified with issue-detail tests and typecheck.

[get6]

Updated the jump control positioning based on UX feedback: it now uses viewport-level positioning () like the AI chat FAB instead of being anchored inside the issue content panel, and stacks just above the chat button. Verified issue-detail tests and typecheck.

[get6]

Correction/clarification: the jump control now uses viewport-level positioning (fixed right-2) like the AI chat FAB instead of being anchored inside the issue content panel, and stacks just above the chat button. Verified issue-detail tests and typecheck.

[get6]

Fine-tuned the jump button relative to the AI chat FAB: kept the vertical stack, and shifted the smaller jump button to right-2.5 so its center aligns with the chat button center (chat FAB is size-10/right-2; jump is size-9/right-2.5). Verified issue-detail tests and typecheck.