Tree layout is determined by BFS rather than DFS.

src/AddIns/Debugger/Debugger.AddIn/Visualizers/Graph/Layout/Tree/TreeLayout.cs

@@ -78,31 +78,48 @@ namespace Debugger.AddIn.Visualizers.Graph.Layout
 
 		void CalculateLayout(PositionedGraph positionedGraph)
 		{
-			HashSet<PositionedNode> seenNodes = new HashSet<PositionedNode>();
-			HashSet<PositionedEdge> treeEdges = new HashSet<PositionedEdge>();
+			// impose a tree structure on the graph
+			HashSet<PositionedEdge> treeEdges = DetermineTreeEdges(positionedGraph.Root);
 			// first layout pass
-			CalculateSubtreeSizes(positionedGraph.Root, seenNodes, treeEdges);
+			CalculateSubtreeSizesRecursive(positionedGraph.Root, treeEdges);
 			// second layout pass
 			CalculateNodePosRecursive(positionedGraph.Root, treeEdges, MarginTop, MarginBottom);
 		}
 		
-		// determines which edges are tree edges, and calculates subtree size for each node
-		private void CalculateSubtreeSizes(PositionedNode root, HashSet<PositionedNode> seenNodes, HashSet<PositionedEdge> treeEdges)
+		
+		HashSet<PositionedEdge> DetermineTreeEdges(PositionedNode root)
 		{
+			var treeEdges = new HashSet<PositionedEdge>();
+			
+			var seenNodes = new HashSet<PositionedNode>();
+			var q = new Queue<PositionedNode>();
+			q.Enqueue(root);
 			seenNodes.Add(root);
-			double subtreeSize = 0;
-			foreach (var property in root.Properties) {
-				var edge = property.Edge;
-				if (edge != null) {
-					var targetNode = edge.Target;
-					if (!seenNodes.Contains(targetNode)) {
-						// when we come to a node for the first time, we declare the incoming edge a tree edge
-						treeEdges.Add(edge);
-						CalculateSubtreeSizes(targetNode, seenNodes, treeEdges);
-						subtreeSize += targetNode.SubtreeSize;
+			
+			while (q.Count > 0) {
+				var node = q.Dequeue();
+				foreach (var property in node.Properties) {
+					var edge = property.Edge;
+					if (edge != null && edge.Target != null) {
+						if (!seenNodes.Contains(edge.Target)) {
+							treeEdges.Add(edge);
+							seenNodes.Add(edge.Target);
+							q.Enqueue(edge.Target);
+						}
 					}
 				}
 			}
+			return treeEdges;
+		}
+		
+		void CalculateSubtreeSizesRecursive(PositionedNode root, HashSet<PositionedEdge> treeEdges)
+		{
+			double subtreeSize = 0;
+			foreach (var child in TreeChildNodes(root, treeEdges)) {
+				CalculateSubtreeSizesRecursive(child, treeEdges);
+				// just sum up the sizes of children
+				subtreeSize += child.SubtreeSize;
+			}
 			root.SubtreeSize = Math.Max(GetLateralSizeWithMargin(root), subtreeSize);
 		}