## Summary
- fix the sticky-bottom state where dragging the scrollbar to the bottom
makes `PageUp` jump to the previous timeline block and then snap
immediately back down
- keep the change scoped to `virtual-follow-list.tsx`, where follow
mode, scroll intent, and bottom pinning are coordinated
## Root Cause
The list only disabled follow mode when it saw an explicit local "user
intent" signal. After reaching the bottom through the native scrollbar,
`PageUp` could move the viewport without tripping that path, so the next
render notification re-enabled the bottom snap immediately.
## Validation
- `npx tsc --noEmit --project packages/ui/tsconfig.json`
- `npm run build --prefix packages/ui`
- manual desktop test: `PageUp` works again from the bottom sticky state
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