Button to jump to first unread message automatically disappears after a short time

### Your use case

#### What would you like to do?
Jump to the first unread message when opening a conversation/room

#### Why would you like to do it?
Convenience.

#### How would you like to achieve it?
Using the button:

Unfortunately, that button disappears after a short amount of time (unexpectedly). In the past, I didn't know that, so thought "okay, I'll open this room, read a bit and can later jump to the first unread message". Then, I saw that the button disappeared.

Now, when opening a room, I am always a bit anxious to click this button in time, before it disappears and I am doomed.

### Have you considered any alternatives?

Manually scrolling up (not so nice in big rooms).

### Additional context

The current behavior seems to be a "feature, not a bug". I consider this a pretty broken UX, though. 🥲

Locate the conversation or room UI that shows the jump-to-first-unread button and reproduce the reported disappearance after opening a room. Trace the behavior that hides the button and verify that users can still jump to the first unread message after reading for a while; the issue does not name a file or test to run.

Written by the indexing model from the issue text.