Restore scroll state after resetting the conversation view's mapping.

// FREEBIE

Signal/src/ViewControllers/ConversationView/MessagesViewController.m

@@ -4436,7 +4436,30 @@ typedef NS_ENUM(NSInteger, MessagesRangeSizeMode) {
     _shouldObserveDBModifications = shouldObserveDBModifications;
 
     if (self.shouldObserveDBModifications) {
+        // We need to call resetMappings when we _resume_ observing DB modifications,
+        // since we've been ignore DB modifications so the mappings can be wrong.
+        //
+        // resetMappings can however have the side effect of increasing the mapping's
+        // "window" size.  If that happens, we need to restore the scroll state.
+
+        // Snapshot the scroll state by measuring the "distance from top of view to
+        // bottom of content"; if the mapping's "window" size grows, it will grow
+        // _upward_.
+        CGFloat viewTopToContentBottom = self.collectionView.contentSize.height - self.collectionView.contentOffset.y;
+
+        NSUInteger oldCellCount = [self.messageMappings numberOfItemsInGroup:self.thread.uniqueId];
         [self resetMappings];
+        NSUInteger newCellCount = [self.messageMappings numberOfItemsInGroup:self.thread.uniqueId];
+
+        // Detect changes in the mapping's "window" size.
+        if (oldCellCount != newCellCount) {
+            // There's no way to imperatively force the collection view to layout its
+            // content, but we can safely use `collectionViewContentSize` to determine
+            // the new content size.
+            CGSize newContentSize = [self.collectionView.collectionViewLayout collectionViewContentSize];
+            CGPoint newContentOffset = CGPointMake(0, MAX(0, newContentSize.height - viewTopToContentBottom));
+            [self.collectionView setContentOffset:newContentOffset animated:NO];
+        }
     }
 }