closes nodebb/nodebb-theme-persona#339

public/src/client/topic.js

@@ -59,8 +59,8 @@ define('forum/topic', [
 		}
 
 		addBlockQuoteHandler();
-
 		addParentHandler();
+		addDropupHandler();
 
 		navigator.init('[component="post"]', ajaxify.data.postcount, Topic.toTop, Topic.toBottom, Topic.navigatorCallback, Topic.calculateIndex);
 
@@ -166,6 +166,17 @@ define('forum/topic', [
 		});
 	}
 
+	function addDropupHandler() {
+		// Locate all dropdowns
+		var target = $('#content .dropdown-menu').parent();
+
+		// Toggle dropup if past 50% of screen
+		$(target).on('show.bs.dropdown', function () {
+			var dropUp = this.getBoundingClientRect().top > ($(window).height() / 2);
+			$(this).toggleClass('dropup', dropUp);
+		});
+	}
+
 	function updateTopicTitle() {
 		var span = components.get('navbar/title').find('span');
 		if ($(window).scrollTop() > 50 && span.hasClass('hidden')) {