What is the safest way to add new messages while keeping the scroll position stable, especially when inserting at the top?

Capture the first visible item (or its index) and offset. Insert new messages using a batch update, then restore scroll position using ScrollTo(savedItem/index, ScrollToPosition.Start, animate: false). This preserves the viewport and avoids jumping.

// before insert
var firstVisibleIndex = args.FirstVisibleItemIndex; // from ItemsViewScrolled
var firstVisibleItem = view.ItemsSource.Cast<object>().ElementAt(firstVisibleIndex);

// insert batch at index 0
items.InsertRange(0, newItems);

// restore
collectionView.ScrollTo(firstVisibleItem, position: ScrollToPosition.Start, animate: false);