As mentioned in the description coalescing mechanism changes slightly and so does user experience. I recommend that reviewers watch recordings attached below:
Current coalescing:

NSE coalescing:

If the UX change originating from this diff is not acceptable the alternative approach we can experiment with is to silence every collapsible notifications and schedule local notifications from the NSE. This way we will be able to set request.identifier property (since we are the ones creating the UNNotificationRequest instance) and let the OS do the coalescing.

Nice! The UI change looks good to me, but letting others review as well.

Nit/offtop: you can use some online video resizer (e.g. this - first link on google) to make video previews smaller because Phabricator isn't able to resize video previews.
Alternatively, you could use a table, It's better but still too large - I mean try this remarkup

| before | after |
| {F754694} | {F754700} |

The UX change doesn't seem particularly bad to me, but I'd like to understand if the approach you're using here (deleting the old notif) is similar to the approaches that other encrypted apps take. Is there no way to set a "collapse ID" from the NSE?

In D9144#269987, @ashoat wrote:

The UX change doesn't seem particularly bad to me, but I'd like to understand if the approach you're using here (deleting the old notif) is similar to the approaches that other encrypted apps take. Is there no way to set a "collapse ID" from the NSE?

I examined NSE code of:

• Signal
• Telegram
• Element

I am not sure of any of those apps actually does some source of notifications coalescing but I am sure that none of them sets identifier property of UNNotificationRequest. However what is most interesting is that Signal appears to actually always call contentHandler with emoty content (silence notification) and then construct local notifications based on data delivered in the UNNotificationContent instance. Therefore if this apps wanted to do coalescing then it can just display new local notification with the same identifier as previous one and obtain the same behaviour as setting apns-collapse-id on the server side. Another interesting this is that Signal seems not to be sending any data in notifications at all. Its NSE contacts their server to fetch actual messages when it receives notification. I am not sure what Telegram and Element do since their source codes are not that easy to understand as Signal',s but I am pretty positive that if they do coalescing they probably also use local notifications.

That said I will allocate 2 - 3 hours of my work time to investigate if is is straightforwardly possible to use local notifications for coalescing in our codebase and update diff it is possible (or not update if reviewers think it is not that necessary).