fix(sdk): Fix day divider removal logic in timeline

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

@@ -700,12 +700,12 @@ impl<'a, 'i> TimelineEventHandler<'a, 'i> {
                         // Pre-requisites for removing the day divider:
                         // 1. there is one preceding the old item at all
                         if self.timeline_items[idx - 1].is_day_divider()
-                            // 2. the next item after the old one being removed
+                            // 2. the item after the old one that was removed
                             //    is virtual (it should be impossible for this
                             //    to be a read marker)
                             && self
                                 .timeline_items
-                                .get(idx + 1)
+                                .get(idx)
                                 .map_or(true, |item| item.is_virtual())
                         {
                             trace!("Removing day divider");

crates/matrix-sdk/src/room/timeline/tests/echo.rs

@@ -136,8 +136,11 @@ async fn remote_echo_new_position() {
 
     // … the local echo should be removed
     assert_matches!(stream.next().await, Some(VecDiff::RemoveAt { index: 1 }));
+    // … along with its day divider
+    assert_matches!(stream.next().await, Some(VecDiff::RemoveAt { index: 0 }));
 
-    // … and the remote echo added
+    // … and the remote echo added (no new day divider because both bob's and
+    // alice's message are from the same day according to server timestamps)
     let item = assert_matches!(stream.next().await, Some(VecDiff::Push { value }) => value);
     assert_matches!(item.as_event().unwrap(), EventTimelineItem::Remote(_));
 }