Fix node drops during LayoutGroup movement

Layout.js

@@ -2,6 +2,7 @@ class Layout {
   constructor(graph) {
     this.graph_ = graph;
 
+    this.nodes_ = [];
     this.nodesByPos_ = new Map();
     this.nodesByGraphNode_ = new Map();
 
@@ -31,9 +32,9 @@ class Layout {
             Math.floor((nodes.length / 2) * SPACING) + (n * SPACING) +
                 (node.subgraph * SPACING * maxRankNodes),
         ];
-        this.nodesByGraphNode_.set(
-            node,
-            new LayoutNode(node, this.nodesByPos_, pos));
+        let layoutNode = new LayoutNode(node, this.nodesByPos_, pos);
+        this.nodes_.push(layoutNode);
+        this.nodesByGraphNode_.set(node, layoutNode);
       }
     }
   }
@@ -65,7 +66,7 @@ class Layout {
   }
 
   iterate() {
-    let objects = Array.from(this.nodesByPos_.values());
+    let objects = Array.from(this.nodes_);
     objects.push(...this.groups_);
     this.setTension(objects);
     this.sortByMostTension(objects);
@@ -127,14 +128,14 @@ class Layout {
   fixOrigin() {
     let min = [Number.MAX_SAFE_INTEGER, Number.MAX_SAFE_INTEGER];
     let max = [Number.MIN_SAFE_INTEGER, Number.MIN_SAFE_INTEGER];
-    for (let node of this.nodesByPos_.values()) {
+    for (let node of this.nodes_) {
       for (let i of [0, 1]) {
         min[i] = Math.min(min[i], node.pos[i]);
         max[i] = Math.max(max[i], node.pos[i]);
       }
     }
     // Offset is negative minimum, e.g min -1 means +1 to all values
-    for (let node of this.nodesByPos_.values()) {
+    for (let node of this.nodes_) {
       for (let i of [0, 1]) {
         node.pos[i] -= min[i];
       }
@@ -153,7 +154,7 @@ class Layout {
       },
     ];
 
-    let nodes = Array.from(this.nodesByPos_.values());
+    let nodes = Array.from(this.nodes_);
     for (let i of [1, 0]) {
       nodes.sort((a, b) => a.pos[i] - b.pos[i]);
     }