Merge pull request unoplatform#16545 from unoplatform/dev/xygu/202405…

…01/droid-combo-flyout-scroll-reset

fix(droid): ComboBox flyout scroll reset on arrange

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_ComboBox.cs

@@ -23,9 +23,6 @@
 using Windows.UI.Input.Preview.Injection;
 using static Private.Infrastructure.TestServices;
 
-
-
-
 #if WINAPPSDK
 using Uno.UI.Extensions;
 #elif __IOS__
@@ -1261,6 +1258,40 @@ public async Task When_Customized_Popup_Placement(PopupPlacementMode mode, doubl
 			}
 		}
 
+#if HAS_UNO
+		[TestMethod]
+		[RunsOnUIThread]
+		public async Task When_ComboPopup_Rearrange_ScrollShouldNotReset()
+		{
+			var SUT = new ComboBox()
+			{
+				ItemsSource = Enumerable.Range(0, 50).Select(x => $"Item {x}").ToArray(),
+				SelectedIndex = 0,
+			};
+			await UITestHelper.Load(SUT);
+
+			// open down-drop
+			SUT.IsDropDownOpen = true;
+			var host = SUT.GetTemplateChild<Border>("PopupBorder") ?? throw new InvalidOperationException("Failed to find Border#PopupBorder");
+			await WindowHelper.WaitForLoaded(host);
+			await UITestHelper.WaitForIdle();
+
+			// scroll to 2 screens away
+			var sv = host.Child as ScrollViewer ?? throw new InvalidOperationException("Failed to find Border#PopupBorder>ScrollViewer");
+			var origin = sv.VerticalOffset;
+			var destination = origin + sv.ViewportHeight * 2;
+			sv.ChangeView(null, verticalOffset: destination, null, disableAnimation: true);
+			await UITestHelper.WaitForIdle();
+			Assert.IsTrue(Math.Abs(destination - sv.VerticalOffset) < 1.0, $"Expect sv.VerticalOffset to be near {destination:0.##}, got: {sv.VerticalOffset:0.##}");
+
+			// force an arrange
+			var cbi = sv.FindFirstChild<ComboBoxItem>();
+			cbi.InvalidateArrange();
+			await UITestHelper.WaitForIdle();
+			Assert.IsTrue(Math.Abs(destination - sv.VerticalOffset) < 1.0, $"Expect sv.VerticalOffset to be still near {destination:0.##}, got: {sv.VerticalOffset:0.##}");
+		}
+#endif
+
 #if __SKIA__ // Requires input injection
 		[TestMethod]
 		[RequiresFullWindow]

src/Uno.UI/UI/Xaml/UIElement.cs

@@ -1096,7 +1096,7 @@ public void Arrange(Rect finalRect)
 			}
 
 			var layouter = ((ILayouterElement)fwe).Layouter;
-			layouter.Arrange(finalRect);
+			layouter.Arrange(finalRect.DeflateBy(fwe.Margin));
 			layouter.ArrangeChild(fwe, finalRect);
 #endif
 		}