From ccd7236346831665c98a1103b98c572cf77844b0 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Tue, 11 Feb 2020 21:59:23 +0100
Subject: [PATCH 1/2] Added failing test for #3550.

Calling `InvalidateMeasure` during `ExecuteInitialLayoutPass` causes invalid bounds/desired size.
---
 .../LayoutManagerTests.cs                     | 33 +++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/tests/Avalonia.Layout.UnitTests/LayoutManagerTests.cs b/tests/Avalonia.Layout.UnitTests/LayoutManagerTests.cs
index 3f20c9a76a..0165b91844 100644
--- a/tests/Avalonia.Layout.UnitTests/LayoutManagerTests.cs
+++ b/tests/Avalonia.Layout.UnitTests/LayoutManagerTests.cs
@@ -374,5 +374,38 @@ namespace Avalonia.Layout.UnitTests
             Assert.True(control.Measured);
             Assert.True(control.IsMeasureValid);
         }
+
+        [Fact]
+        public void Calling_ExecuteLayoutPass_From_ExecuteInitialLayoutPass_Does_Not_Break_Measure()
+        {
+            // Test for issue #3550.
+            var control = new LayoutTestControl();
+            var root = new LayoutTestRoot { Child = control };
+            var count = 0;
+
+            root.LayoutManager.ExecuteInitialLayoutPass(root);
+            control.Measured = false;
+
+            control.DoMeasureOverride = (l, s) =>
+            {
+                if (count++ == 0)
+                {
+                    control.InvalidateMeasure();
+                    root.LayoutManager.ExecuteLayoutPass();
+                    return new Size(100, 100);
+                }
+                else
+                {
+                    return new Size(200, 200);
+                }
+            };
+
+            root.InvalidateMeasure();
+            control.InvalidateMeasure();
+            root.LayoutManager.ExecuteInitialLayoutPass(root);
+
+            Assert.Equal(new Size(200, 200), control.Bounds.Size);
+            Assert.Equal(new Size(200, 200), control.DesiredSize);
+        }
     }
 }

From d5fc004df0d149452e873040102f1e98cb929a2d Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Tue, 11 Feb 2020 21:59:51 +0100
Subject: [PATCH 2/2] Prevent reentrancy in ExecuteInitialLayoutPass.

Fixes #3550.
---
 src/Avalonia.Layout/LayoutManager.cs | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/src/Avalonia.Layout/LayoutManager.cs b/src/Avalonia.Layout/LayoutManager.cs
index 1792655a13..69a4406828 100644
--- a/src/Avalonia.Layout/LayoutManager.cs
+++ b/src/Avalonia.Layout/LayoutManager.cs
@@ -132,8 +132,16 @@ namespace Avalonia.Layout
         /// <inheritdoc/>
         public void ExecuteInitialLayoutPass(ILayoutRoot root)
         {
-            Measure(root);
-            Arrange(root);
+            try
+            {
+                _running = true;
+                Measure(root);
+                Arrange(root);
+            }
+            finally
+            {
+                _running = false;
+            }
 
             // Running the initial layout pass may have caused some control to be invalidated
             // so run a full layout pass now (this usually due to scrollbars; its not known