Fixes the AlignmentDialogPopupPositioner margins

DialogHost.Avalonia/DialogOverlayPopupHost.axaml.cs

@@ -86,6 +86,15 @@ public void Hide()
             _root.Children.Remove(this);
         }
 
+        protected override Size MeasureOverride(Size availableSize)
+        {
+            if (PopupPositioner is IDialogPopupPositionerConstrainable constrainable)
+            {
+                return base.MeasureOverride(constrainable.Constrain(availableSize));
+            }
+            return base.MeasureOverride(availableSize);
+        }
+
         /// <inheritdoc />
         protected override void ArrangeCore(Rect finalRect) {
             var margin = Margin;
@@ -100,8 +109,8 @@ protected override void ArrangeCore(Rect finalRect) {
             var positioner = PopupPositioner ?? CenteredDialogPopupPositioner.Instance;
             var bounds = positioner.Update(size, contentSize);
 
-            ArrangeOverride(bounds.Size).Constrain(size);
-            Bounds = new Rect(bounds.X + margin.Left, bounds.Y + margin.Top, bounds.Width, bounds.Height);
+            var (finalWidth, finalHeight) = ArrangeOverride(bounds.Size).Constrain(size);
+            Bounds = new Rect(bounds.X + margin.Left, bounds.Y + margin.Top, finalWidth, finalHeight);
         }
 
 

DialogHost.Avalonia/Positioners/AlignmentDialogPopupPositioner.cs

@@ -8,7 +8,7 @@ namespace DialogHostAvalonia.Positioners {
     /// <remarks>
     /// Default values for <see cref="HorizontalAlignment"/> and <see cref="VerticalAlignment"/> is <c>Stretch</c> and it will be act TopLeft alignment
     /// </remarks>
-    public class AlignmentDialogPopupPositioner : AvaloniaObject, IDialogPopupPositioner {
+    public class AlignmentDialogPopupPositioner : AvaloniaObject, IDialogPopupPositioner, IDialogPopupPositionerConstrainable {
         public static readonly StyledProperty<HorizontalAlignment> HorizontalAlignmentProperty
             = Layoutable.HorizontalAlignmentProperty.AddOwner<AlignmentDialogPopupPositioner>();
 
@@ -46,5 +46,11 @@ public Rect Update(Size anchorRectangle, Size size) {
             var aligned = rect.Align(constrainRect, GetValue(HorizontalAlignmentProperty), GetValue(VerticalAlignmentProperty));
             return new Rect(margin.Left + aligned.Left, margin.Top + aligned.Top, aligned.Width, aligned.Height);
         }
+
+        /// <inheritdoc />
+        public Size Constrain(Size availableSize)
+        {
+            return availableSize.Deflate(Margin);
+        }
     }
 }

DialogHost.Avalonia/Positioners/IDialogPopupPositionerConstrainable.cs

@@ -0,0 +1,17 @@
+using Avalonia;
+
+namespace DialogHostAvalonia.Positioners
+{
+    /// <summary>
+    /// Implement this for your positioner if it constrains the popup, e.g. margin
+    /// </summary>
+    public interface IDialogPopupPositionerConstrainable
+    {
+        /// <summary>
+        /// Constrain the space available for popup
+        /// </summary>
+        /// <param name="availableSize">Initial size</param>
+        /// <returns>Constrained size</returns>
+        public Size Constrain(Size availableSize);
+    }
+}