Scroll to bottom when adding a new message (#1450)

Added a comment to try to make this easier to understand as you read it.

FREEBIE

js/views/message_list_view.js

@@ -48,7 +48,12 @@
             this.$el.scrollTop(this.scrollPosition - this.$el.outerHeight());
         },
         scrollToBottomIfNeeded: function() {
-            if (!this.atBottom()) {
+            // This is counter-intuitive. Our current bottomOffset is reflective of what
+            //   we last measured, not necessarily the current state. And this is called
+            //   after we just made a change to the DOM: inserting a message, or an image
+            //   finished loading. So if we were near the bottom before, we _need_ to be
+            //   at the bottom again. So we scroll to the bottom.
+            if (this.atBottom()) {
                 this.scrollToBottom();
             }
         },