Merge remote-tracking branch 'origin/fix/human-transcript-projection' into fix/web-transcript-projection

# Conflicts:
#	apps/web/tests/snapshots/seeded-history/ui.expected.md
#	packages/client/runtime/README.i18n.yaml
#	packages/client/runtime/README.md
#	packages/client/runtime/src/client/index.ts
#	packages/client/runtime/src/client/sessions/fold-adapter.ts
#	packages/client/runtime/tests/fold-adapter.spec.ts
#	packages/client/ui-trajectory/src/client/layout.ts
#	packages/client/ui-trajectory/src/client/spans.ts
#	packages/compact/compact/README.i18n.yaml

packages/client/ui-conversation/tests/chat-view.spec.tsx

@@ -371,6 +371,42 @@ describe('ChatView', () => {
     expect(view.queryByLabelText('回到底部')).toBeNull()
   })
 
+  it('entering the at-bottom threshold does not snap the remaining scroll distance', () => {
+    const h = makeHarness({ nodes: [user(1, 'q'), assistant(2, 'a')] })
+    const view = render(<h.ChatView {...h.props} />)
+    const scroller = view.container.querySelector('[class*="scroll"]') as HTMLDivElement
+    Object.defineProperty(scroller, 'scrollHeight', { value: 1000, writable: true })
+    Object.defineProperty(scroller, 'clientHeight', { value: 300, writable: true })
+    // Inside FOLLOW_THRESHOLD (24) but not flush with the floor — the chrome
+    // re-render from setAtBottom must not force scrollTop to scrollHeight.
+    scroller.scrollTop = 690 // distance-to-bottom = 10
+    fireEvent.scroll(scroller)
+    expect(view.queryByLabelText('回到底部')).toBeNull()
+    expect(scroller.scrollTop).toBe(690)
+  })
+
+  it('under data-conversation-scroll, bottom-follow targets the host scrollport', () => {
+    const host = document.createElement('div')
+    host.setAttribute('data-conversation-scroll', '')
+    Object.defineProperty(host, 'scrollHeight', { value: 2000, writable: true, configurable: true })
+    Object.defineProperty(host, 'clientHeight', { value: 500, writable: true, configurable: true })
+    Object.defineProperty(host, 'scrollTop', { value: 0, writable: true, configurable: true })
+    document.body.appendChild(host)
+    try {
+      const h = makeHarness({ nodes: [user(1, 'q'), assistant(2, 'a')] })
+      const view = render(<h.ChatView {...h.props} />, { container: host })
+      // Open jump uses the host, not the local .scroll node.
+      expect(host.scrollTop).toBe(2000)
+      host.scrollTop = 100
+      fireEvent.scroll(host)
+      expect(view.getByLabelText('回到底部')).toBeTruthy()
+      fireEvent.click(view.getByLabelText('回到底部'))
+      expect(host.scrollTop).toBe(2000)
+    } finally {
+      host.remove()
+    }
+  })
+
   it('paging button loads older and shows its busy label', () => {
     const h = makeHarness({ nodes: [user(5, 'later')], hasMore: true })
     const view = render(<h.ChatView {...h.props} />)