Call ApplyStyling unconditionally in MeasureCore

src/Avalonia.Base/Layout/Layoutable.cs

@@ -543,6 +543,8 @@ protected static void AffectsArrange<T>(params AvaloniaProperty[] properties)
         /// </remarks>
         protected virtual Size MeasureCore(Size availableSize)
         {
+            ApplyStyling();
+
             if (IsVisible)
             {
                 var margin = Margin;
@@ -555,7 +557,6 @@ protected virtual Size MeasureCore(Size availableSize)
                     margin = LayoutHelper.RoundLayoutThickness(margin, scale);
                 }
 
-                ApplyStyling();
                 ApplyTemplate();
 
                 var minMax = new MinMax(this);

src/Avalonia.Controls/StackPanel.cs

@@ -261,15 +261,16 @@ protected override Size MeasureOverride(Size availableSize)
                 // Get next child.
                 var child = children[i];
 
+                // Measure the child.
+                child.Measure(layoutSlotSize);
+
                 bool isVisible = child.IsVisible;
 
                 if (isVisible && !hasVisibleChild)
                 {
                     hasVisibleChild = true;
                 }
 
-                // Measure the child.
-                child.Measure(layoutSlotSize);
                 Size childDesiredSize = child.DesiredSize;
 
                 // Accumulate child size.

tests/Avalonia.Base.UnitTests/Layout/MeasureTests.cs

@@ -1,10 +1,39 @@
 using Avalonia.Controls;
+using Avalonia.Styling;
+using Avalonia.UnitTests;
 using Xunit;
 
 namespace Avalonia.Base.UnitTests.Layout
 {
     public class MeasureTests
     {
+        [Fact]
+        public void Style_Hiding_Control_Should_Be_Applied_Before_Measuring()
+        {
+            var child = new Border
+            {
+                Width = 100,
+                Height = 100,
+                Classes = { "hidden" }
+            };
+            var target = new Decorator
+            {
+                Child = child
+            };
+            var root = new TestRoot(target);
+
+            root.Styles.Add(new Style(x => x.OfType<Border>().Class("hidden"))
+            {
+                Setters = { new Setter(Visual.IsVisibleProperty, false) }
+            });
+
+            target.Measure(Size.Infinity);
+
+            Assert.False(child.IsVisible);
+            Assert.Equal(new Size(0,0), child.DesiredSize);
+            Assert.Equal(new Size(0,0), target.DesiredSize);
+        }
+
         [Fact]
         public void Margin_Should_Be_Included_In_DesiredSize()
         {

tests/Avalonia.Controls.UnitTests/StackPanelTests.cs

@@ -1,5 +1,6 @@
 using System.Linq;
 using Avalonia.Layout;
+using Avalonia.Styling;
 using Avalonia.UnitTests;
 using Xunit;
 
@@ -330,6 +331,39 @@ public void Spacing_Not_Added_For_Invisible_Children(Orientation orientation)
             Assert.Equal(sizeWithTwoChildren, sizeWithThreeChildren);
         }
 
+        [Theory]
+        [InlineData(Orientation.Horizontal)]
+        [InlineData(Orientation.Vertical)]
+        public void Spacing_Not_Added_For_Children_Hidden_By_Style_Applied_During_Measure(Orientation orientation)
+        {
+            var target = new StackPanel
+            {
+                Spacing = 40,
+                Orientation = orientation,
+                Children =
+                {
+                    new StackPanel { Width = 10, Height = 10, Classes = { "hidden" } },
+                    new StackPanel { Width = 10, Height = 10 },
+                    new StackPanel { Width = 10, Height = 10 },
+                }
+            };
+
+            var root = new TestRoot(target);
+
+            root.Styles.Add(new Style(x => x.OfType<StackPanel>().Class("hidden"))
+            {
+                Setters = { new Setter(Visual.IsVisibleProperty, false) }
+            });
+
+            target.Measure(Size.Infinity);
+
+            var expected = orientation == Orientation.Horizontal ?
+                new Size(60, 10) :
+                new Size(10, 60);
+
+            Assert.Equal(expected, target.DesiredSize);
+        }
+
         [Theory]
         [InlineData(Orientation.Horizontal)]
         [InlineData(Orientation.Vertical)]

tests/Avalonia.Controls.UnitTests/WindowTests.cs

@@ -1642,6 +1642,83 @@ public void WindowDecorationsTheme_Should_Apply_To_Decorations()
             }
         }
 
+        [Fact]
+        public void IsVisible_Setter_Should_Affect_Measurements_Inside_WindowDrawnDecorationsContent()
+        {
+            using var app = UnitTestApplication.Start(TestServices.StyledWindow);
+
+            var windowImpl = MockWindowingPlatform.CreateWindowMock();
+            windowImpl.Setup(x => x.NeedsManagedDecorations).Returns(true);
+            windowImpl.Setup(x => x.RequestedDrawnDecorations).Returns(PlatformRequestedDrawnDecoration.TitleBar);
+
+            var window = new Window(windowImpl.Object);
+
+            var stackPanel = new StackPanel
+            {
+                Width = 32,
+                Spacing = 2,
+                Children =
+                {
+                    new Control { Height = 32 },
+                    new Control
+                    {
+                        Height = 32,
+                        Classes = { "hidden-by-style" }
+                    },
+                }
+            };
+
+            var contentControl = new ContentControl
+            {
+                Content = new Control
+                {
+                    Height = 32,
+                    Width = 32,
+                    Classes = { "hidden-by-style" }
+                }
+            };
+
+            var content = new WindowDrawnDecorationsContent
+            {
+                Overlay = new ContentControl
+                {
+                    Content = new Panel
+                    {
+                        Children = { stackPanel, contentControl }
+                    }
+                }
+            };
+
+            var template = new WindowDrawnDecorationsTemplate
+            {
+                Content = (IServiceProvider? _) => new TemplateResult<WindowDrawnDecorationsContent>(content, new NameScope())
+            };
+
+            var theme = new ControlTheme(typeof(WindowDrawnDecorations))
+            {
+                Setters =
+                {
+                    new Setter(WindowDrawnDecorations.TemplateProperty, template)
+                }
+            };
+
+            var style = new Style(x => x.Is<WindowDrawnDecorations>().Template().OfType<Control>().Class("hidden-by-style"))
+            {
+                Setters =
+                {
+                    new Setter(Visual.IsVisibleProperty, false)
+                }
+            };
+
+            window.WindowDecorationsTheme = theme;
+            window.Styles.Add(style);
+            window.Show();
+            window.Measure(Size.Infinity);
+
+            Assert.Equal(new Size(), contentControl.DesiredSize);
+            Assert.Equal(new Size(32, 32), stackPanel.DesiredSize);
+        }
+
         public class TitleBarDecorationsTests : ScopedTestBase
         {
             private static Window CreateWindowWithDrawnDecorations(PlatformAllowedWindowActions allowedActions = PlatformAllowedWindowActions.All)