Select the last (top) overlapping object when showing the popup

When there are many overlapping objects, pick the last one which
is drawn at the top of the others. This is the same way the
_getNodeAt() / _getEdgeAt() methods of the SelectionMixin work.

lib/network/Network.js

@@ -1336,34 +1336,44 @@ Network.prototype._checkShowPopup = function (pointer) {
   if (this.popupObj == undefined) {
     // search the nodes for overlap, select the top one in case of multiple nodes
     var nodes = this.nodes;
+    var overlappingNodes = [];
     for (id in nodes) {
       if (nodes.hasOwnProperty(id)) {
         var node = nodes[id];
         if (node.isOverlappingWith(obj)) {
           if (node.getTitle() !== undefined) {
-            this.popupObj = node;
-            break;
+            overlappingNodes.push(id);
           }
-          // if you hover over a node, the title of the edge is not supposed to be shown.
-          nodeUnderCursor = true;
         }
       }
     }
+
+    if (overlappingNodes.length > 0) {
+      // if there are overlapping nodes, select the last one, this is the
+      // one which is drawn on top of the others
+      this.popupObj = this.nodes[overlappingNodes[overlappingNodes.length - 1]];
+      // if you hover over a node, the title of the edge is not supposed to be shown.
+      nodeUnderCursor = true;
+    }
   }
 
   if (this.popupObj === undefined && nodeUnderCursor == false) {
     // search the edges for overlap
     var edges = this.edges;
+    var overlappingEdges = [];
     for (id in edges) {
       if (edges.hasOwnProperty(id)) {
         var edge = edges[id];
         if (edge.connected && (edge.getTitle() !== undefined) &&
             edge.isOverlappingWith(obj)) {
-          this.popupObj = edge;
-          break;
+          overlappingEdges.push(id);
         }
       }
     }
+
+    if (overlappingEdges.length > 0) {
+      this.popupObj = this.edges[overlappingEdges[overlappingEdges.length - 1]];
+    }
   }
 
   if (this.popupObj) {