Handle ScrollContentPresenter extent rounding errors

samples/Sandbox/MainWindow.axaml

@@ -1,11 +1,4 @@
 <Window xmlns="https://github.com/avaloniaui"
         xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
         x:Class="Sandbox.MainWindow">
-        
-        <ScrollViewer>
-                <StackPanel>
-                        <Button Margin="0 100000000000000000 0 0">0</Button>
-                        <Button>1</Button>
-                </StackPanel>
-        </ScrollViewer>
 </Window>

src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs

@@ -473,18 +473,31 @@ namespace Avalonia.Controls.Presenters
             }
 
             Viewport = finalSize;
+            Extent = ComputeExtent(finalSize);
+            _isAnchorElementDirty = true;
 
+            return finalSize;
+        }
+
+        private Size ComputeExtent(Size viewportSize)
+        {
             var childMargin = Child!.Margin;
+
             if (Child.UseLayoutRounding)
             {
                 var scale = LayoutHelper.GetLayoutScale(Child);
                 childMargin = LayoutHelper.RoundLayoutThickness(childMargin, scale, scale);
             }
 
-            Extent = Child!.Bounds.Size.Inflate(childMargin);
-            _isAnchorElementDirty = true;
+            var extent = Child!.Bounds.Size.Inflate(childMargin);
 
-            return finalSize;
+            if (MathUtilities.AreClose(extent.Width, viewportSize.Width, LayoutHelper.LayoutEpsilon))
+                extent = extent.WithWidth(viewportSize.Width);
+
+            if (MathUtilities.AreClose(extent.Height, viewportSize.Height, LayoutHelper.LayoutEpsilon))
+                extent = extent.WithHeight(viewportSize.Height);
+
+            return extent;
         }
 
         private void OnScrollGesture(object? sender, ScrollGestureEventArgs e)

tests/Avalonia.Controls.UnitTests/Presenters/ScrollContentPresenterTests.cs

@@ -275,6 +275,37 @@ namespace Avalonia.Controls.UnitTests.Presenters
             Assert.Equal(new Size(203.2, 203.2), target.Extent);
         }
 
+        [Fact]
+        public void Extent_Should_Be_Rounded_To_Viewport_When_Close()
+        {
+            var root = new TestRoot
+            {
+                LayoutScaling = 1.75,
+                UseLayoutRounding = true
+            };
+
+            var target = new ScrollContentPresenter
+            {
+                HorizontalAlignment = HorizontalAlignment.Center,
+                VerticalAlignment = VerticalAlignment.Center,
+                Content = new Border
+                {
+                    Width = 164.57142857142858,
+                    Height = 164.57142857142858,
+                    Margin = new Thickness(6)
+                }
+            };
+
+            root.Child = target;
+            target.UpdateChild();
+            target.Measure(new Size(1000, 1000));
+            target.Arrange(new Rect(0, 0, 1000, 1000));
+
+            Assert.Equal(new Size(176.00000000000003, 176.00000000000003), target.Child!.DesiredSize);
+            Assert.Equal(new Size(176, 176), target.Viewport);
+            Assert.Equal(new Size(176, 176), target.Extent);
+        }
+
         [Fact]
         public void Extent_Width_Should_Be_Arrange_Width_When_CanScrollHorizontally_False()
         {