The cumulative offset of the container is irrelevant

Since we are only comparing the cumulative offset of the position of the
images that live inside the IFRAME, it doesn't matter where the IFRAME
lives on the parent page.

imp/js/imp.js

@@ -189,8 +189,8 @@ var IMP_JS = {
         mb_height = mb.getHeight();
 
         /* Load messages within 1 scrolled page of range boundaries. */
-        range_top = mb.scrollTop + mb.cumulativeOffset().top - mb_height;
-        range_bottom = mb.scrollTop + mb.cumulativeOffset().top + (2 * mb_height);
+        range_top = mb.scrollTop - mb_height;
+        range_bottom = mb.scrollTop + (2 * mb_height);
 
         imgs = Prototype.Selector.select('IMG[data-src]', this.iframeDoc(iframe)).findAll(Element.visible);