Separate messages after 10-min gap, with a sender row

[gnprice]

When someone sends a message, and then sends another message a few hours later and there are no other messages shown between those, our current design makes this look just the same as if the second message were just a minute after the first. In particular, the only timestamp in sight may be the one from the older message.

This can be confusing. For example:

Somebody wrote a message, then several hours later wrote "Dinner in 17 minutes". That message appeared to have a timestamp of hours before in the UI.

(If the reader suspects this might be what they're looking at, they do have the ability to check: long-pressing the later message will show its timestamp as part of the message action sheet. That was part of #217. But there's currently nothing in the normal message-list UI that hints at whether this is the case.)

To fix this, let's add separation when consecutive messages in the message list are far apart in time. Specifically:

• We'll use a threshold of 10 minutes.
• When a message is at least 10 minutes later than the previous one, we'll show a sender row above it, even if we otherwise wouldn't do so. (This will include the timestamp, as well as the other items that normally go in the same row with a message timestamp: the sender's avatar and name.)

References

• Chat thread: #mobile > Timestamp for consecutive messages @ 💬
• Related issue: Show message timestamps on swipe #457
• Related issue: msglist: Message bounds hard to see in a run of messages from the same user #1142