messages sent from push notifs reliably appear out of order

they get appended to the timeline before the sync responses that they were in response to, so causality is always reversed. instead there should be no local echo for stuff sent from nse

Start by tracing the iOS push-notification handling in the notification service extension (NSE) and how its messages are appended relative to sync responses. Done means messages sent from the NSE have no local echo and the timeline no longer reverses their causal order.

Written by the indexing model from the issue text.