MDL-44844 core: Fix custom menu navigation

Atto introduced a rule to hide all Nodes with the yui3-menu-hidden class
but this rule is too general and applies to non-atto content too.

As a result, when loading the menu and during the XY calculation, the
native Element.getBoundingClientRect() function was returning a bounding
box size of 0 because the Node was not visible and the node was being
hidden from view when it was later made visible.

lib/editor/atto/styles.css

@@ -155,7 +155,7 @@ div.editor_atto_content:hover .atto_control {
     display: block;
 }
 
-.yui3-menu-hidden {
+.editor_atto_menu.yui3-menu-hidden {
     display: none;
 }