Consider scrolling to bottom of conversation view on first load in session

[eloquence]

If you select a source for the first time in a given session, we currently put you at the top of the conversation on the initial load. We scroll to the bottom when you send a reply, and we preserve any change to the scrolling position if you switch between sources.

We have first evidence that putting you at the top of the conversation view can be frustrating for long conversations (imagine coming back to the same source session after session, having to scroll to the bottom each time). Compare it with the behavior in Signal Desktop, for example, which will always put you at the bottom when you load a conversation.

We should consider implementing the same logic when you first click a source in a session, i.e. always putting the user at the bottom on the initial load of a conversation in that session.

Acceptance Criteria

• Loading a conversation for the first time in a given session should put you at the bottom of the conversation view
• Any changes to the scroll position for a conversation should continue to be remembered during the session

See also

Previous discussions in #688, #653, and #685

[eloquence]

Some alternatives that could be considered to solve for the same issue:

1. Prominent "jump to bottom" link (debatable whether that's a solution, user might still be frustrated by having to do so every time)
2. Ordering option (could be very disorienting as it does not match typical messaging app behavior; could also be confusing on a system used by more than 1 user until we have proper ways to track user-specific prefs)
3. Wait until we have the ability to identify content new to a given user and then use that status to decide where to scroll (that is the assumption in Highlight sources with unseen source submissions #187 -- it describes a feature of scrolling "new activity" into view)

[sssoleileraaa]

3. Wait until we have the ability to identify content new to a given user and then use that status to decide where to scroll (that is the assumption in Highlight sources with unseen source submissions #187 -- it describes a feature of scrolling "new activity" into view)

Update: The backend for account-level read/unread, which we're now calling seen/unseen, will set us up nicely to implement number 3 here, when we're ready to prioritize it.

1. Prominent "jump to bottom" link (debatable whether that's a solution, user might still be frustrated by having to do so every time)
   This is still a useful feature and can be made into a separate issue.

[ninavizz]

This is a common pattern in messaging clients (how all of them work, actually), and I would advocate having this implemented.