Added invalidation to LayoutThickness property in Border and ContentPresenter and implemented minor review fixes.

src/Avalonia.Controls/Border.cs

@@ -1,8 +1,10 @@
+using System;
 using Avalonia.Collections;
 using Avalonia.Controls.Shapes;
 using Avalonia.Controls.Utils;
 using Avalonia.Layout;
 using Avalonia.Media;
+using Avalonia.Utilities;
 using Avalonia.VisualTree;
 
 namespace Avalonia.Controls
@@ -88,6 +90,18 @@ namespace Avalonia.Controls
             AffectsMeasure<Border>(BorderThicknessProperty);
         }
 
+        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        {
+            base.OnPropertyChanged(change);
+            switch (change.Property.Name)
+            {
+                case nameof(UseLayoutRounding):
+                case nameof(BorderThickness):
+                    _layoutThickness = null;
+                    break;
+            }
+        }
+
         /// <summary>
         /// Gets or sets a brush with which to paint the background.
         /// </summary>
@@ -169,29 +183,39 @@ namespace Avalonia.Controls
             set => SetValue(BoxShadowProperty, value);
         }
 
-        private Thickness _layoutThickness = default;
+        private Thickness? _layoutThickness;
+        private double _scale;
 
         private Thickness LayoutThickness
         {
             get
             {
-                if (_layoutThickness == default)
+                VerifyScale();
+
+                if (_layoutThickness == null)
                 {
                     var borderThickness = BorderThickness;
 
                     if (UseLayoutRounding)
-                    {
+                        borderThickness = LayoutHelper.RoundLayoutThickness(BorderThickness, _scale, _scale);
-                        var scale = LayoutHelper.GetLayoutScale(this);
-                        borderThickness = LayoutHelper.RoundLayoutThickness(BorderThickness, scale, scale);
-                    }
 
                     _layoutThickness = borderThickness;
                 }
 
-                return _layoutThickness;
+                return _layoutThickness.Value;
             }
         }
 
+        private void VerifyScale()
+        {
+            var currentScale = LayoutHelper.GetLayoutScale(this);
+            if (MathUtilities.AreClose(currentScale, _scale))
+                return;
+
+            _scale = currentScale;
+            _layoutThickness = null;
+        }
+
         /// <summary>
         /// Renders the control.
         /// </summary>

src/Avalonia.Controls/Presenters/ContentPresenter.cs

@@ -8,6 +8,7 @@ using Avalonia.Layout;
 using Avalonia.LogicalTree;
 using Avalonia.Media;
 using Avalonia.Metadata;
+using Avalonia.Utilities;
 
 namespace Avalonia.Controls.Presenters
 {
@@ -249,6 +250,10 @@ namespace Avalonia.Controls.Presenters
                 case nameof(TemplatedParent):
                     TemplatedParentChanged(change);
                     break;
+                case nameof(UseLayoutRounding):
+                case nameof(BorderThickness):
+                    _layoutThickness = null;
+                    break;
             }
         }
 
@@ -329,29 +334,39 @@ namespace Avalonia.Controls.Presenters
             InvalidateMeasure();
         }
 
-        private Thickness _layoutThickness = default;
+        private Thickness? _layoutThickness;
+        private double _scale;
 
         private Thickness LayoutThickness
         {
             get
             {
-                if (_layoutThickness == default)
+                VerifyScale();
+
+                if (_layoutThickness == null)
                 {
                     var borderThickness = BorderThickness;
 
                     if (UseLayoutRounding)
-                    {
+                        borderThickness = LayoutHelper.RoundLayoutThickness(BorderThickness, _scale, _scale);
-                        var scale = LayoutHelper.GetLayoutScale(this);
-                        borderThickness = LayoutHelper.RoundLayoutThickness(BorderThickness, scale, scale);
-                    }
 
                     _layoutThickness = borderThickness;
                 }
 
-                return _layoutThickness;
+                return _layoutThickness.Value;
             }
         }
 
+        private void VerifyScale()
+        {
+            var currentScale = LayoutHelper.GetLayoutScale(this);
+            if (MathUtilities.AreClose(currentScale, _scale))
+                return;
+
+            _scale = currentScale;
+            _layoutThickness = null;
+        }
+
         /// <inheritdoc/>
         public override void Render(DrawingContext context)
         {

src/Avalonia.Layout/Layoutable.cs

@@ -653,7 +653,7 @@ namespace Avalonia.Layout
                 // Margin has to be treated separately because the layout rounding function is not linear
                 // f(a + b) != f(a) + f(b)
                 // If the margin isn't pre-rounded some sizes will be offset by 1 pixel in certain scales.
-                if (UseLayoutRounding)
+                if (useLayoutRounding)
                 {
                     margin = LayoutHelper.RoundLayoutThickness(margin, scale, scale);
                 }