Don't remeasure the HScrollView during the layout pass

src/Controls/samples/Controls.Sample.UITests/Issues/Issue20920.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Microsoft.Maui.Controls;
+
+namespace Maui.Controls.Sample.Issues
+{
+	[Issue(IssueTracker.None, 20920, "Nested ScrollView does not work in Android", PlatformAffected.Android)]
+	public class Issue20920 : TestContentPage
+	{
+		public static Label DestructorCount = new Label() { Text = "0" };
+		protected override void Init()
+		{
+			Content = new ScrollView()
+            {
+                Orientation = Microsoft.Maui.ScrollOrientation.Horizontal,
+                Content = new ScrollView()
+                {
+                    Orientation = Microsoft.Maui.ScrollOrientation.Vertical,
+                    Content = new Image()
+                    {
+                        Margin = 100,
+                        AutomationId = "dotnet_bot",
+                        Source = "dotnet_bot.png",
+                        HeightRequest = 1000,
+                        WidthRequest = 1000
+                    }
+                }
+            };
+		}
+	}
+}

src/Controls/tests/UITests/Tests/Issues/Issue20920.cs

@@ -0,0 +1,29 @@
+using System.Drawing;
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.AppiumTests.Issues;
+
+public class Issue20920 : _IssuesUITest
+{
+	public Issue20920(TestDevice device) : base(device) { }
+
+	public override string Issue => "Nested ScrollView does not work in Android";
+
+	[Test]
+	public void ScrollingBothDirectionsWithNestedScrollViews()
+	{
+ 		// TODO: Correct this test for other platforms
+ 		this.IgnoreIfPlatforms(new TestDevice[] { TestDevice.Mac, TestDevice.Windows, TestDevice.iOS });
+		var initialPosition = App.WaitForElement("dotnet_bot").GetRect();
+
+		App.ScrollDown("dotnet_bot");
+		App.ScrollRight("dotnet_bot");
+
+		var afterScrollPosition = App.WaitForElement("dotnet_bot").GetRect();
+
+		Assert.Less(afterScrollPosition.X, initialPosition.X);
+		Assert.Less(afterScrollPosition.Y, initialPosition.Y);
+	}
+}

src/Core/src/Platform/Android/MauiScrollView.cs

@@ -85,6 +85,7 @@ public void SetContent(View content)
 
 		public void SetOrientation(ScrollOrientation orientation)
 		{
+			bool orientationChanged = _scrollOrientation != orientation;
 			_scrollOrientation = orientation;
 
 			if (orientation == ScrollOrientation.Horizontal || orientation == ScrollOrientation.Both)
@@ -109,6 +110,12 @@ public void SetOrientation(ScrollOrientation orientation)
 
 					AddView(_hScrollView);
 				}
+				// If the user has changed between horiztonal and both we want to request a new layout
+				// so the Horizontal Layout can be adjusted to satisfy the new orientation.
+				else if(orientationChanged)
+				{
+					PlatformInterop.RequestLayoutIfNeeded(this);
+				}
 			}
 			else
 			{
@@ -143,7 +150,7 @@ public override bool OnInterceptTouchEvent(MotionEvent? ev)
 
 		public override bool OnTouchEvent(MotionEvent? ev)
 		{
-			if (ev == null || !Enabled)
+			if (ev == null || !Enabled || _scrollOrientation == ScrollOrientation.Neither)
 				return false;
 
 			if (ShouldSkipOnTouch)
@@ -184,6 +191,25 @@ void IScrollBarView.AwakenScrollBars()
 
 		bool IScrollBarView.ScrollBarsInitialized { get; set; }
 
+		protected override void OnMeasure(int widthMeasureSpec, int heightMeasureSpec)
+		{
+			base.OnMeasure(widthMeasureSpec, heightMeasureSpec);
+
+			// If we have bidirectional scrolling then we can just let everything flow through naturally.
+			// The HorizontalScrollView will automatically size its height to the content and thus enable
+			// vertical scolling
+			// If we're only enabling horizontal scrolling then we want to force the horizontal scrollView
+			// to be the same size as the NestedScrollView this way it can't be scrolled vertically
+			if (_hScrollView?.Parent == this && _content is not null && !_isBidirectional)
+			{
+				var hScrollViewHeight = this.MeasuredHeight;
+				var hScrollViewWidth = this.MeasuredWidth;
+
+				_hScrollView.Measure(MeasureSpec.MakeMeasureSpec(hScrollViewWidth, MeasureSpecMode.Exactly),
+					MeasureSpec.MakeMeasureSpec(hScrollViewHeight, MeasureSpecMode.Exactly));
+			}
+		}
+
 		protected override void OnLayout(bool changed, int left, int top, int right, int bottom)
 		{
 			base.OnLayout(changed, left, top, right, bottom);
@@ -197,12 +223,6 @@ protected override void OnLayout(bool changed, int left, int top, int right, int
 				//if we are scrolling both ways we need to lay out our MauiHorizontalScrollView with more than the available height
 				//so its parent the NestedScrollView can scroll vertically
 				hScrollViewHeight = _isBidirectional ? Math.Max(hScrollViewHeight, scrollViewContentHeight) : hScrollViewHeight;
-
-				// Ensure that the horizontal scrollview has been measured at the actual target size
-				// so that it updates its internal bookkeeping to use the full size
-				_hScrollView.Measure(MeasureSpec.MakeMeasureSpec(right - left, MeasureSpecMode.Exactly),
-					MeasureSpec.MakeMeasureSpec(hScrollViewHeight, MeasureSpecMode.Exactly));
-
 				_hScrollView.Layout(0, 0, hScrollViewWidth, hScrollViewHeight);
 			}
 

src/Core/src/PublicAPI/net-android/PublicAPI.Unshipped.txt

@@ -65,6 +65,7 @@ override Microsoft.Maui.Layouts.FlexBasis.Equals(object? obj) -> bool
 override Microsoft.Maui.Layouts.FlexBasis.GetHashCode() -> int
 override Microsoft.Maui.MauiAppCompatActivity.DispatchTouchEvent(Android.Views.MotionEvent? e) -> bool
 override Microsoft.Maui.Platform.ContentViewGroup.GetClipPath(int width, int height) -> Android.Graphics.Path?
+override Microsoft.Maui.Platform.MauiScrollView.OnMeasure(int widthMeasureSpec, int heightMeasureSpec) -> void
 override Microsoft.Maui.Platform.NavigationViewFragment.OnDestroy() -> void
 override Microsoft.Maui.PlatformContentViewGroup.JniPeerMembers.get -> Java.Interop.JniPeerMembers!
 override Microsoft.Maui.PlatformContentViewGroup.ThresholdClass.get -> nint