FIX: Fallback to last post when the in-view post cannot be detected.

In certain situations the detection method fails. For example, it fails
for a topic containing: a long post (~1600px height), gap, small action
(focus on this), hidden reply.

app/assets/javascripts/discourse/lib/keyboard-shortcuts.js.es6

@@ -445,6 +445,9 @@ export default {
       $selected = $articles
         .toArray()
         .find(article => article.getBoundingClientRect().top > offset);
+      if (!$selected) {
+        $selected = $articles[$articles.length - 1];
+      }
       direction = 0;
     }