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/OnPlatformTests.cs

@@ -178,5 +178,28 @@ public void StackLayoutOrientation()
 			layout = new StackLayout().LoadFromXaml(xaml);
 			Assert.AreEqual(StackOrientation.Horizontal, layout.Orientation);
 		}
+
+		[Test]
+		public void OnIdiomReturnsBindablePropertyDefaultWhenNotSet()
+		{
+			var xaml = @"
+			<Button
+				xmlns=""http://schemas.microsoft.com/dotnet/2021/maui""
+				xmlns:x=""http://schemas.microsoft.com/winfx/2009/xaml""
+				Text=""Sample Button""
+				HeightRequest=""{OnIdiom Tablet=50}"" />";
+
+			mockDeviceInfo.Idiom = DeviceIdiom.Tablet;
+			var button = new Button().LoadFromXaml(xaml);
+			Assert.AreEqual(50, button.HeightRequest);
+
+			foreach (var idiom in new[] { DeviceIdiom.Phone, DeviceIdiom.Desktop, DeviceIdiom.TV, DeviceIdiom.Watch })
+			{
+				mockDeviceInfo.Idiom = idiom;
+				button = new Button().LoadFromXaml(xaml);
+				Assert.AreEqual(-1, button.HeightRequest, $"Expected default value -1 for idiom {idiom}");
+			}
+		}
+
 	}
 }