Improve how the fetch_status.has_found_newest is set when loading a view from cache

[timabbott]

This logic is wrong/not ideal, in that it doesn't check whether fetch_status.has_found_newest is true.

export function process_visible() {
    if (
        message_lists.current !== undefined &&
        viewport_is_visible_and_focused() &&
        message_viewport.bottom_rendered_message_visible() &&
        message_lists.current.view.is_fetched_end_rendered()
    ) {
        process_scrolled_to_bottom();
    }
}

See a2fe33a for detailed context on why the state of affairs is what it is today.

Fixing this likely involves extending the fetch_status structure to be capable of tracking the state of "We think we have all the latest messages, but are asking the server to double-check our understanding anyway."

See https://chat.zulip.org/#narrow/stream/9-issues/topic/message.20list.20doesn't.20load.20newer.20messages.20when.20near.20end/near/1795139 for detailed context.

We could also think about working towards revisiting the behavior of unconditionally always asking the server about older messages in a given conversation; if we're able to clean out certain categories of bugs, in theory that should not be necessary a lot of the time thanks to extensive local caching.

[zulipbot]

Hello @zulip/server-refactoring members, this issue was labeled with the "area: refactoring" label, so you may want to check it out!