Fix: RelativePanel calculation error with extensible and aligned child (#19474)

* Feat: Add unit test for RelativePanel.
The ScrollViewer's scrollbar does not display correctly.

* Fix: RelativePanel calculation error
The ScrollViewer's scrollbar does not display correctly.

src/Avalonia.Controls/RelativePanel.cs

@@ -333,6 +333,22 @@ namespace Avalonia.Controls
                         : node.AlignBottomWithNode.Bottom * 0.5;
                 }
 
+                if (node.BelowNode != null)
+                {
+                    if (node.Top.IsNaN())
+                    {
+                        node.Top = AvailableSize.Height - node.BelowNode.Bottom;
+                    }
+                }
+                
+                if (node.RightOfNode != null)
+                {
+                    if (node.Left.IsNaN())
+                    {
+                        node.Left = AvailableSize.Width - node.RightOfNode.Right;
+                    }
+                }
+                
                 var availableHeight = AvailableSize.Height - node.Top - node.Bottom;
                 if (availableHeight.IsNaN())
                 {
@@ -383,10 +399,6 @@ namespace Avalonia.Controls
                         node.Right = node.RightOfNode.Right - childSize.Width;
                     }
 
-                    if (node.Left.IsNaN())
-                    {
-                        node.Left = AvailableSize.Width - node.RightOfNode.Right;
-                    }
                 }
 
                 if (node.BelowNode != null)
@@ -396,10 +408,6 @@ namespace Avalonia.Controls
                         node.Bottom = node.BelowNode.Bottom - childSize.Height;
                     }
 
-                    if (node.Top.IsNaN())
-                    {
-                        node.Top = AvailableSize.Height - node.BelowNode.Bottom;
-                    }
                 }
 
                 if (node.AlignHorizontalCenterWith != null)

tests/Avalonia.Markup.Xaml.UnitTests/Xaml/RelativePanelTests.cs

@@ -0,0 +1,116 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using System.Reactive.Subjects;
+using System.Threading;
+using Avalonia.Controls;
+using Avalonia.Data.Converters;
+using Avalonia.UnitTests;
+using Xunit;
+
+namespace Avalonia.Markup.Xaml.UnitTests.Xaml
+{
+    public class RelativePanelTests : XamlTestBase
+    {
+        [Fact]
+        public void ScrollViewer_Viewport_Small_Than_Bounds()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+        xmlns:local='clr-namespace:Avalonia.Markup.Xaml.UnitTests.Xaml;assembly=Avalonia.Markup.Xaml.UnitTests'
+        Height='800'
+        Width='1000'
+>
+  <RelativePanel x:Name=""TestRelativePanel"">
+    <Panel
+          x:Name=""Area1""
+          RelativePanel.AlignTopWithPanel=""True""
+          RelativePanel.AlignLeftWithPanel=""True""
+          RelativePanel.AlignRightWithPanel=""True""
+          Height=""100""
+          Background=""LightSkyBlue"">
+      <TextBlock
+          Text=""Area1""
+          HorizontalAlignment=""Center""
+          VerticalAlignment=""Center""
+          />
+      <!-- <Button Click=""Button_OnClick"">Second</Button> -->
+    </Panel>
+    <Panel
+      x:Name=""Area2""
+      RelativePanel.Below=""Area1""
+      RelativePanel.AlignLeftWithPanel=""True""
+      RelativePanel.AlignBottomWithPanel=""True""
+      Background=""DeepSkyBlue""
+        Width=""100""
+      >
+      <TextBlock
+        Text=""Area2""
+        HorizontalAlignment=""Center""
+        VerticalAlignment=""Center""
+        Height=""100""
+      />
+    </Panel>
+    <ScrollViewer
+        x:Name=""TestArea""
+        Background=""Aqua""
+        RelativePanel.Below=""Area1""
+        RelativePanel.RightOf=""Area2""
+        RelativePanel.AlignRightWithPanel=""True""
+        RelativePanel.AlignBottomWithPanel=""True""
+        HorizontalScrollBarVisibility=""Visible""
+        Margin=""0 0 0 0""
+        >
+      <ItemsControl
+        ItemsSource=""{Binding DataExample}""
+        >
+        <ItemsControl.ItemsPanel>
+          <ItemsPanelTemplate>
+            <StackPanel></StackPanel>
+          </ItemsPanelTemplate>
+        </ItemsControl.ItemsPanel>
+        <ItemsControl.ItemTemplate>
+          <DataTemplate>
+            <StackPanel Orientation=""Horizontal"">
+              <TextBlock Width=""100"" Text=""{Binding Id}""></TextBlock>
+              <TextBlock Width=""100"" Text=""{Binding Id}""></TextBlock>
+              <TextBlock Width=""100"" Text=""{Binding Id}""></TextBlock>
+              <TextBlock Width=""100"" Text=""{Binding Id}""></TextBlock>
+              <TextBlock Width=""100"" Text=""{Binding Id}""></TextBlock>
+              <TextBlock Width=""100"" Text=""{Binding Id}""></TextBlock>
+              <TextBlock Width=""100"" Text=""{Binding Id}""></TextBlock>
+              <TextBlock Width=""100"" Text=""{Binding Id}""></TextBlock>
+              <TextBlock Width=""100"" Text=""{Binding Id}""></TextBlock>
+              <TextBlock Width=""100"" Text=""{Binding Id}""></TextBlock>
+            </StackPanel>
+          </DataTemplate>
+        </ItemsControl.ItemTemplate>
+      </ItemsControl>
+    </ScrollViewer>
+  </RelativePanel>
+</Window>";
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+                {
+                    var panel = window.FindControl<RelativePanel>("TestRelativePanel");
+
+                    panel.DataContext = new
+                    {
+                        DataExample = Enumerable.Range(1001, 100).Select(e => new { Id = $"{e}" })
+                    };
+                }
+                window.ApplyTemplate();
+                window.Show();
+                    
+                var sv = window.FindControl<ScrollViewer>("TestArea");
+                Assert.True(sv.Viewport.Width < sv.Bounds.Width);
+                Assert.True(sv.Viewport.Height < sv.Bounds.Height);
+            }
+        }
+
+    }
+}