Fix multiple ScrollContentPresenter.BringDescendantIntoView calls (#22001)

* Add failing test for ScrollContentPresenter

* Fix ScrollContentPresenter multiple BringDescendantIntoView

* Update src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs

Co-authored-by: Steven Kirk <grokys@users.noreply.github.com>

---------

Co-authored-by: Steven Kirk <grokys@users.noreply.github.com>

src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs

@@ -251,17 +251,20 @@ namespace Avalonia.Controls.Presenters
                 return scrollable.BringIntoView(control, targetRect);
             }
 
-            var transform = target.TransformToVisual(this);
-
-            if (transform == null)
+            // The `viewport` rectangle computed below is in extent coordinates, so transform
+            // `targetRect` into that space too. Going via Child rather than via this + Offset
+            // keeps the result independent of Offset, which may have changed since the last
+            // arrange.
+            if (target.TransformToVisual(Child) is not { } transform)
             {
                 return false;
             }
 
-            transform *= Matrix.CreateTranslation(Offset);
-
-            var rectangle = targetRect.TransformToAABB(transform.Value);
-            Rect viewport = new Rect(Offset.X, Offset.Y, Viewport.Width, Viewport.Height);
+            var childPadding = GetChildPadding();
+            var childMargin = GetChildMargin();
+            var childContentOrigin = new Vector(childPadding.Left + childMargin.Left, childPadding.Top + childMargin.Top);
+            var rectangle = targetRect.TransformToAABB(transform).Translate(childContentOrigin);
+            var viewport = new Rect(Offset.X, Offset.Y, Viewport.Width, Viewport.Height);
 
             double minX = ComputeScrollOffsetWithMinimalScroll(viewport.Left, viewport.Right, rectangle.Left, rectangle.Right);
             double minY = ComputeScrollOffsetWithMinimalScroll(viewport.Top, viewport.Bottom, rectangle.Top, rectangle.Bottom);
@@ -528,16 +531,26 @@ namespace Avalonia.Controls.Presenters
             return padding + borderThickness;
         }
 
-        private Size ComputeExtent(Size viewportSize, Thickness padding)
+        private Thickness GetChildMargin()
         {
-            var childMargin = Child!.Margin;
+            if (Child is not { } child)
+                return default;
+
+            var margin = child.Margin;
 
             if (Child.UseLayoutRounding)
             {
                 var scale = LayoutHelper.GetLayoutScale(Child);
-                childMargin = LayoutHelper.RoundLayoutThickness(childMargin, scale);
+                margin = LayoutHelper.RoundLayoutThickness(margin, scale);
             }
 
+            return margin;
+        }
+
+        private Size ComputeExtent(Size viewportSize, Thickness padding)
+        {
+            var childMargin = GetChildMargin();
+
             var extent = Child!.Bounds.Size.Inflate(childMargin).Inflate(padding);
 
             if (MathUtilities.AreClose(extent.Width, viewportSize.Width, LayoutHelper.LayoutEpsilon))

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

@@ -475,6 +475,37 @@ namespace Avalonia.Controls.UnitTests.Presenters
             Assert.Equal(new Vector(100, 100), target.Offset);
         }
 
+        [Fact]
+        public void BringDescendantIntoView_Should_Be_Idempotent_Before_Next_Layout_Pass()
+        {
+            var panel = new StackPanel();
+
+            for (var i = 0; i < 100; ++i)
+                panel.Children.Add(new Border { Height = 20 });
+
+            var target = new ScrollContentPresenter
+            {
+                Width = 50,
+                Height = 100,
+                CanVerticallyScroll = true,
+                Content = panel,
+            };
+
+            target.UpdateChild();
+            target.Measure(Size.Infinity);
+            target.Arrange(new Rect(0, 0, 50, 100));
+
+            // The 50th child spans 1000..1020, so with a 100px viewport it is brought into view by scrolling to 920.
+            var child = panel.Children[50];
+
+            target.BringDescendantIntoView(child, new Rect(child.Bounds.Size));
+            Assert.Equal(920, target.Offset.Y);
+            Assert.False(target.IsArrangeValid);
+
+            target.BringDescendantIntoView(child, new Rect(child.Bounds.Size));
+            Assert.Equal(920, target.Offset.Y);
+        }
+
         [Fact]
         public void BringDescendantIntoView_Should_Handle_Child_Margin()
         {

tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs

@@ -1724,7 +1724,7 @@ namespace Avalonia.Controls.UnitTests
         [InlineData(0.5d,
             0, 7,
             0, 7,
-            7, 17)]
+            0, 9)]
         public void Focused_Container_Is_Positioned_Correctly_when_Container_Size_Change_Causes_It_To_Be_Moved_Into_Visible_Viewport(double bufferFactor, 
             int firstIndex1, int lastIndex1,
             int firstIndex2, int lastIndex2,