Fix multi-root traversal

tree.go

@@ -49,10 +49,14 @@ func (tree *TreeWidget) Draw(buffer *termui.Buffer) {
 
 // AddRootNode adds a new top-level node to the tree
 func (tree *TreeWidget) AddRootNode(node *Node) {
-	tree.rootNodes = append(tree.rootNodes, node)
-	if len(tree.rootNodes) == 1 {
-		tree.selectedNode = node.children[0]
+	if len(tree.rootNodes) > 0 {
+		node.prev = tree.rootNodes[len(tree.rootNodes)-1]
+		node.prev.next = node
+	} else if len(tree.rootNodes) == 0 {
+		// If this is our first node, then we need to mark this as the current node
+		tree.selectedNode = node
 	}
+	tree.rootNodes = append(tree.rootNodes, node)
 	node.setTree(tree)
 }