From 51f698d5959c234e4729236b8436b41add58689e Mon Sep 17 00:00:00 2001
From: Charlie Kindel <charlie@kindel.com>
Date: Mon, 8 Jun 2020 20:15:48 -0600
Subject: [PATCH 1/3] fixed

---
 Terminal.Gui/Core/View.cs      | 12 ++++++------
 UICatalog/Scenarios/Generic.cs | 16 ++++++++++++++++
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/Terminal.Gui/Core/View.cs b/Terminal.Gui/Core/View.cs
index c612d16b8..be4260f32 100644
--- a/Terminal.Gui/Core/View.cs
+++ b/Terminal.Gui/Core/View.cs
@@ -1385,10 +1385,12 @@ namespace Terminal.Gui {
 				}
 			}
 
-			// if graph has edges then
 			if (edges.Any ()) {
-				// return error (graph has at least one cycle)
-				return null;
+				if (!object.ReferenceEquals(edges.First ().From, edges.First ().To)) {
+					throw new InvalidOperationException ($"TopologicalSort (for Pos/Dim) cannot find {edges.First ().From}. Did you forget to add it to {this}?");
+				} else {
+					throw new InvalidOperationException ("TopologicalSort encountered a recursive cycle in the relative Pos/Dim in the views of " + this);
+				}
 			} else {
 				// return L (a topologically sorted order)
 				return result;
@@ -1454,8 +1456,6 @@ namespace Terminal.Gui {
 			}
 
 			var ordered = TopologicalSort (nodes, edges);
-			if (ordered == null)
-				throw new Exception ("There is a recursive cycle in the relative Pos/Dim in the views of " + this);
 
 			foreach (var v in ordered) {
 				if (v.LayoutStyle == LayoutStyle.Computed)
@@ -1483,7 +1483,7 @@ namespace Terminal.Gui {
 		/// <param name="hotPos">The returning hot-key position.</param>
 		/// <param name="showHotKey">The character immediately to the right relative to the hot-key position</param>
 		/// <returns>It aims to facilitate the preparation for <see cref="TextAlignment"/> procedures.</returns>
-		public virtual ustring GetTextFromHotKey(ustring text, Rune hotKey, out int hotPos, out Rune showHotKey)
+		public virtual ustring GetTextFromHotKey (ustring text, Rune hotKey, out int hotPos, out Rune showHotKey)
 		{
 			Rune hot_key = (Rune)0;
 			int hot_pos = -1;
diff --git a/UICatalog/Scenarios/Generic.cs b/UICatalog/Scenarios/Generic.cs
index 5502958bf..dce4153b4 100644
--- a/UICatalog/Scenarios/Generic.cs
+++ b/UICatalog/Scenarios/Generic.cs
@@ -12,6 +12,22 @@ namespace UICatalog {
 				Y = Pos.Center (),
 				Clicked = () => MessageBox.Query (20, 7, "Hi", "Neat?", "Yes", "No")
 			});
+
+			var label = new Label ("Label: ") { 
+				X = 0,
+				Y = 0
+			};
+			Win.Add (label);
+
+			var button = new Button ("test") {
+				X = Pos.Y (label),
+				Y = Pos.Bottom (label),
+			};
+			button.Width = Dim.Width (label);
+			label.X = button.X;
+
+
+			Win.Add (button);
 		}
 	}
 }
\ No newline at end of file

From 3ab20b658d4e07ebd8b3cf149fd06e5b71420248 Mon Sep 17 00:00:00 2001
From: Charlie Kindel <charlie@kindel.com>
Date: Mon, 8 Jun 2020 20:29:09 -0600
Subject: [PATCH 2/3] unit tests for topologicalsort errors

---
 UnitTests/ViewTests.cs | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/UnitTests/ViewTests.cs b/UnitTests/ViewTests.cs
index 059544090..f89cbf6d3 100644
--- a/UnitTests/ViewTests.cs
+++ b/UnitTests/ViewTests.cs
@@ -107,5 +107,33 @@ namespace Terminal.Gui {
 
 			// TODO: Add more
 		}
+
+		[Fact]
+		public void TopologicalSort_Missing_Add ()
+		{
+			var root = new View ();
+			var sub1 = new View ();
+			root.Add (sub1);
+			var sub2 = new View ();
+			sub1.Width = Dim.Width(sub2);
+
+			Assert.Throws<InvalidOperationException> (() => root.LayoutSubviews ());
+
+			sub2.Width = Dim.Width (sub1);
+
+			Assert.Throws<InvalidOperationException> (() => root.LayoutSubviews ());
+		}
+
+		[Fact]
+		public void TopologicalSort_Recursive_Ref ()
+		{
+			var root = new View ();
+			var sub1 = new View ();
+			root.Add (sub1);
+			var sub2 = new View ();
+			root.Add (sub2);
+			sub2.Width = Dim.Width (sub2);
+			Assert.Throws<InvalidOperationException> (() => root.LayoutSubviews ());
+		}
 	}
 }

From 290a693097c80378b8c0e01bd851112839e2c10e Mon Sep 17 00:00:00 2001
From: Charlie Kindel <charlie@kindel.com>
Date: Mon, 8 Jun 2020 20:32:03 -0600
Subject: [PATCH 3/3] back out change to generic scenario

---
 UICatalog/Scenarios/Generic.cs | 16 ----------------
 1 file changed, 16 deletions(-)

diff --git a/UICatalog/Scenarios/Generic.cs b/UICatalog/Scenarios/Generic.cs
index dce4153b4..5502958bf 100644
--- a/UICatalog/Scenarios/Generic.cs
+++ b/UICatalog/Scenarios/Generic.cs
@@ -12,22 +12,6 @@ namespace UICatalog {
 				Y = Pos.Center (),
 				Clicked = () => MessageBox.Query (20, 7, "Hi", "Neat?", "Yes", "No")
 			});
-
-			var label = new Label ("Label: ") { 
-				X = 0,
-				Y = 0
-			};
-			Win.Add (label);
-
-			var button = new Button ("test") {
-				X = Pos.Y (label),
-				Y = Pos.Bottom (label),
-			};
-			button.Width = Dim.Width (label);
-			label.X = button.X;
-
-
-			Win.Add (button);
 		}
 	}
 }
\ No newline at end of file