Fix unread count separator and mark read when viewing behavior.

Fixes #12510

app/src/main/java/org/thoughtcrime/securesms/conversation/ConversationFragment.java

@@ -1380,7 +1380,7 @@ public class ConversationFragment extends LoggingFragment implements Multiselect
     }
 
     int position = getListLayoutManager().findFirstVisibleItemPosition();
-    if (position == getListAdapter().getItemCount() - 1) {
+    if (position == -1 || position == getListAdapter().getItemCount() - 1) {
       return;
     }
 

app/src/main/java/org/thoughtcrime/securesms/conversation/LastSeenHeader.java

@@ -30,7 +30,7 @@ class LastSeenHeader extends StickyHeaderDecoration {
 
   @Override
   protected boolean hasHeader(RecyclerView parent, StickyHeaderAdapter stickyAdapter, int position) {
-    if (lastSeenTimestamp <= 0) {
+    if (lastSeenTimestamp <= 0 || unreadCount <= 0) {
       return false;
     }