ui: Transfer reply details from old to new item when receiving dup event

crates/matrix-sdk-ui/src/timeline/event_handler.rs

@@ -936,6 +936,8 @@ impl<'a> TimelineEventHandler<'a> {
                     // TODO: Check whether anything is different about the
                     //       old and new item?
 
+                    transfer_details(&mut item, &old_item);
+
                     let old_item_id = old_item.internal_id;
 
                     if idx == self.state.items.len() - 1
@@ -1135,6 +1137,24 @@ impl<'a> TimelineEventHandler<'a> {
     }
 }
 
+/// Transfer `TimelineDetails` that weren't available on the original item and
+/// have been fetched separately (only `reply_to` for now) from `old_item` to
+/// `item`, given two items for an event that was re-received.
+///
+/// `old_item` *should* always be a local echo usually, but with the sliding
+/// sync proxy, we often re-receive remote events that aren't remote echoes.
+fn transfer_details(item: &mut EventTimelineItem, old_item: &EventTimelineItem) {
+    let TimelineItemContent::Message(msg) = &mut item.content else { return };
+    let TimelineItemContent::Message(old_msg) = &old_item.content else { return };
+
+    let Some(in_reply_to) = &mut msg.in_reply_to else { return };
+    let Some(old_in_reply_to) = &old_msg.in_reply_to else { return };
+
+    if matches!(&in_reply_to.event, TimelineDetails::Unavailable) {
+        in_reply_to.event = old_in_reply_to.event.clone();
+    }
+}
+
 pub(crate) fn update_read_marker(
     items: &mut ObservableVector<Arc<TimelineItem>>,
     fully_read_event: Option<&EventId>,