use updated translation key and add a comment

customize.dist/src/less2/include/toolbar.less

@@ -971,6 +971,14 @@
             }
         }
         button.cp-toolbar-collapse {
+/*
+        XXX this button toggles between two states and uses two titles for either state.
+        tippy doesn't work well with this, so the titles are applied to two icons
+        and we display the relevant one. Unfortunately, the hoverable area for the inner icon
+        is very small, which makes it unlikely that you'll see the title unless
+        you hover perfectly. The same is true of the drive's grid/list view toggle.
+*/
+
             .fa {
                 margin-right: 0px;
             }

www/common/toolbar.js

@@ -456,12 +456,12 @@ MessengerUI, Messages, Pages) {
     };
 
     //Messages.collapse = Messages.admin_support_collapse;
-    Messages.toolbar_collapse = "Collapse toolbar"; // XXX
-    Messages.toolbar_expand = "Expand toolbar"; // XXX
+    Messages.ui_collapse = "Collapse toolbar"; // XXX
+    Messages.ui_expand = "Expand toolbar"; // XXX
 
     createCollapse = function (toolbar) {
-        var up = h('i.fa.fa-chevron-up', {title: Messages.toolbar_collapse});
-        var down = h('i.fa.fa-chevron-down', {title: Messages.toolbar_expand});
+        var up = h('i.fa.fa-chevron-up', {title: Messages.ui_collapse});
+        var down = h('i.fa.fa-chevron-down', {title: Messages.ui_expand});
 
         var $button = $(h('button.cp-toolbar-collapse',[
             up,