Fixed the propagation of the background event to its children.

js/image_viewer.js

@@ -15,6 +15,9 @@ function pop(event) {
 }
 
 function unpop(event) {
+    if (event.target.id == 'prev' || event.target.id == 'next' || event.target.nodeName == 'IMG') {
+        return;
+    }
     var viewer = document.getElementById('viewer');
     viewer.classList.add('hidden');
     if (viewer.lastChild.nodeName == 'IMG') {