Avoid scrolling when moving focus from a treelist widget (#1489929)

program/js/treelist.js

@@ -755,7 +755,7 @@ function rcube_treelist_widget(node, p)
       case 9:  // Tab
         // jump to last/first item to move focus away from the treelist widget by tab
         var limit = rcube_event.get_modifier(e) == SHIFT_KEY ? 'first' : 'last';
-        container.find('li[role=treeitem]:has(a)')[limit]().find('a:'+limit).focus();
+        focus_noscroll(container.find('li[role=treeitem]:has(a)')[limit]().find('a:'+limit));
         break;
     }
 
@@ -788,6 +788,19 @@ function rcube_treelist_widget(node, p)
     }
   }
 
+  /**
+   * Focus the given element without scrolling the list container
+   */
+  function focus_noscroll(elem)
+  {
+    if (elem.length) {
+      var frame = container.parent().get(0) || { scrollTop:0 },
+        y = frame.scrollTop || frame.scrollY;
+      elem.focus();
+      frame.scrollTop = y;
+    }
+  }
+
 
   ///// drag & drop support