Fix: OnIdiom Default Value Handling for ValueType Properties

src/Controls/src/Xaml/MarkupExtensions/OnIdiomExtension.cs

@@ -61,7 +61,18 @@ public object ProvideValue(IServiceProvider serviceProvider)
 
 			var value = GetValue();
 			if (value == null && propertyType.IsValueType)
+			{
+				if(bp != null)
+				{
+					object targetObject = valueProvider.TargetObject;
+
+						if (targetObject is Setter)
+							return null;
+						else
+							return bp.GetDefaultValue(targetObject as BindableObject);
+				}
 				return Activator.CreateInstance(propertyType);
+			}
 
 			if (Converter != null)
 				return Converter.Convert(value, propertyType, ConverterParameter, CultureInfo.CurrentUICulture);

src/Controls/tests/TestCases.HostApp/Issues/Issue9978.xaml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Controls.TestCases.HostApp.Issues.Issue9978"
+             Title="Issue9978">
+            <VerticalStackLayout>
+                <Image AutomationId="MauiImage" Source="groceries.png" HeightRequest="{OnIdiom  Tablet=100}" />
+            </VerticalStackLayout>
+</ContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue9978.xaml.cs

@@ -0,0 +1,11 @@
+namespace Controls.TestCases.HostApp.Issues;
+
+[Issue(IssueTracker.Github, 9978, "VisualElement.HeightRequest defaults to 0 instead of -1 when using OnIdiom default value",
+		PlatformAffected.All)]
+public partial class Issue9978 : ContentPage
+{
+	public Issue9978()
+	{
+		InitializeComponent();
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue9978.cs

@@ -0,0 +1,24 @@
+using System;
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Tests.Issues;
+
+public class Issue9978 : _IssuesUITest
+{
+	public Issue9978(TestDevice device) : base(device)
+	{
+	}
+
+	public override string Issue => "VisualElement.HeightRequest defaults to 0 instead of -1 when using OnIdiom default value";
+
+
+    [Test]
+	[Category(UITestCategories.Layout)]
+	public void Issue9978Test()
+	{
+		App.WaitForElement("MauiImage");
+
+	}
+}

src/Controls/tests/Xaml.UnitTests/Issues/Issue9978.xaml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Microsoft.Maui.Controls.Xaml.UnitTests.Issues.Issue9978"
+             Title="Issue9978">
+    <Button x:Name="mauiButton" Text="Sample Button" HeightRequest="{OnIdiom Tablet=50}" /> 
+</ContentPage>

src/Controls/tests/Xaml.UnitTests/Issues/Issue9978.xaml.cs

@@ -0,0 +1,52 @@
+using Microsoft.Maui.Controls.Core.UnitTests;
+using Microsoft.Maui.Devices;
+using NUnit.Framework;
+
+namespace Microsoft.Maui.Controls.Xaml.UnitTests.Issues;
+
+public partial class Issue9978 : ContentPage
+{
+	public Issue9978()
+	{
+		InitializeComponent();
+	}
+
+	public Issue9978(bool useCompiledXaml)
+	{
+		//this stub will be replaced at compile time
+	}
+
+	[TestFixture]
+	public class Tests
+	{
+		[TestCase(false)]
+		[TestCase(true)]
+		public void OnIdiomReturnsBindableDefaultIfNotSet(bool useCompiledXaml)
+		{
+			var idioms = new[]
+			{
+				DeviceIdiom.Tablet,   
+				DeviceIdiom.Phone,     
+				DeviceIdiom.Desktop,
+				DeviceIdiom.Watch,
+				DeviceIdiom.TV         
+			};
+
+			foreach (var idiom in idioms)
+			{
+				DeviceInfo.SetCurrent(new MockDeviceInfo { Idiom = idiom });
+				var page = new Issue9978(useCompiledXaml);
+
+				if (idiom == DeviceIdiom.Tablet)
+				{
+					Assert.AreEqual(50, page.mauiButton.HeightRequest, $"Expected 50 for idiom Tablet");
+				}
+				else
+				{
+					Assert.AreEqual(-1, page.mauiButton.HeightRequest, $"Expected -1 for idiom {idiom}");
+				}
+			}
+		}
+
+	}
+}

src/Controls/tests/Xaml.UnitTests/OnPlatformTests.cs

@@ -178,5 +178,6 @@ public void StackLayoutOrientation()
 			layout = new StackLayout().LoadFromXaml(xaml);
 			Assert.AreEqual(StackOrientation.Horizontal, layout.Orientation);
 		}
+
 	}
 }