Add cycle detection to tree rendering

src/Spectre.Console/Widgets/CircularTreeException.cs

@@ -0,0 +1,15 @@
+using System;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// Indicates that the tree being rendered includes a cycle, and cannot be rendered.
+    /// </summary>
+    public sealed class CircularTreeException : Exception
+    {
+        internal CircularTreeException(string message)
+            : base(message)
+        {
+        }
+    }
+}

src/Spectre.Console/Widgets/Tree.cs

@@ -6,7 +6,8 @@ using Spectre.Console.Rendering;
 namespace Spectre.Console
 {
     /// <summary>
-    /// A renderable tree.
+    /// Representation of non-circular tree data.
+    /// Each node added to the tree may only be present in it a single time, in order to facilitate cycle detection.
     /// </summary>
     public sealed class Tree : Renderable, IHasTreeNodes
     {
@@ -54,6 +55,7 @@ namespace Spectre.Console
         protected override IEnumerable<Segment> Render(RenderContext context, int maxWidth)
         {
             var result = new List<Segment>();
+            var visitedNodes = new HashSet<TreeNode>();
 
             var stack = new Stack<Queue<TreeNode>>();
             stack.Push(new Queue<TreeNode>(new[] { _root }));
@@ -77,6 +79,10 @@ namespace Spectre.Console
 
                 var isLastChild = stackNode.Count == 1;
                 var current = stackNode.Dequeue();
+                if (!visitedNodes.Add(current))
+                {
+                    throw new CircularTreeException("Cycle detected in tree - unable to render.");
+                }
 
                 stack.Push(stackNode);