Maintain scroll position in case of Load position ABOVE messages.

Account for message headers generated when messages are loaded above a
list of messages.

This is important because message headers are also counted as items in the
recyclerview and hence when retaining the position of visible message at
the top of list, we need to account for all the message headers generated
for the set of LoadPosition.ABOVE messages.

Fixes #353.

Author: Sam1301

app/src/main/java/com/zulip/android/activities/MessageListFragment.java

@@ -435,8 +435,16 @@ public void onMessages(Message[] messages, LoadPosition pos,
 
         if (pos == LoadPosition.ABOVE) {
             adapter.setHeaderShowing(moreAbove);
+            /* if we don't account for message headers generated, a scroll jitter is observed
+             whenever messages are loaded ABOVE as the original top message's position is not
+             retained. We also need to account for the message header of original top message in
+             order to retain the correct scroll position.
+            */
+
             // Restore the position of the top item
-            this.recyclerView.scrollToPosition(topPosBefore + addedCount);
+            // account for generated message headers
+            // +1 for the header of top item
+            this.recyclerView.scrollToPosition(topPosBefore + addedCount + headerParents + 1);
 
             if (noFurtherMessages) {
                 loadedToTop = true;