[iOS] Scroll-to-bottom arrow flashes in DM subthreads while already at bottom

## Bug description

In the Buzz mobile app on iOS, the **scroll to bottom** arrow appears inside direct-message subthreads and repeatedly flashes even when the subthread is already scrolled to the bottom.

## Steps to reproduce

1. Open the Buzz mobile app on iOS.
2. Open a direct message conversation.
3. Open a subthread on a message.
4. Scroll to the bottom of the subthread, or open it when already positioned at the bottom.
5. Observe the scroll-to-bottom arrow.

## Actual behavior

The arrow remains visible or repeatedly appears and disappears, producing a flashing effect despite there being no newer content below the current position.

## Expected behavior

The scroll-to-bottom arrow should remain hidden while the subthread is at the bottom. It should appear only when additional content exists below the current scroll position, without flickering as layout or scroll measurements settle.

## Environment

- App: Buzz mobile app
- Platform: iOS
- Buzz version: Not provided
- iOS version/device: Not provided