fix: CalendarView maxdate scrolling

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

@@ -24,6 +24,8 @@
 [RunsOnUIThread]
 public class Given_CalendarView
 {
+	const int DEFAULT_MIN_MAX_DATE_YEAR_OFFSET = 100;
+
 	[TestMethod]
 	[UnoWorkItem("https://github.com/unoplatform/uno/issues/16123")]
 	[Ignore("Test is unstable on CI: https://github.com/unoplatform/uno/issues/16123")]
@@ -61,6 +63,42 @@
 		await UITestHelper.Load(calendarView);
 	}
 
+	[TestMethod]
+	public async Task When_Scroll_To_MaxDate()
+	{
+		var calendarView = new CalendarView()
+		{
+			DisplayMode = CalendarViewDisplayMode.Decade
+		};
+
+		await UITestHelper.Load(calendarView);
+
+		Type calendarViewType = typeof(CalendarView);
+		MethodInfo ChangeVisualStateInfo = calendarViewType.GetMethod("ChangeVisualState", BindingFlags.NonPublic | BindingFlags.Instance);
+
+		// Scroll to max date
+		calendarView.SetDisplayDate(calendarView.MaxDate);
+
+		// Switch to Year view
+		calendarView.DisplayMode = CalendarViewDisplayMode.Year;
+		ChangeVisualStateInfo.Invoke(calendarView, new object[] { false });
+		await TestServices.WindowHelper.WaitForIdle();
+
+		// Switch back to Decade view
+		calendarView.DisplayMode = CalendarViewDisplayMode.Decade;
+		ChangeVisualStateInfo.Invoke(calendarView, new object[] { false });
+		await TestServices.WindowHelper.WaitForIdle();
+
+		// Decade viewport should be full of items (no missing row)
+		calendarView.GetActiveGeneratorHost(out var pHost);
+		var maxDecadeIndex = DEFAULT_MIN_MAX_DATE_YEAR_OFFSET * 2;
+		var maxDisplayedItems = pHost.Panel.Rows * pHost.Panel.Cols;
+
+		// The first visible index should be less than the max possible index minus the max items we can display
+		// Worst case scenario is that the last row only has 1 item
+		Assert.IsTrue(pHost.Panel.FirstVisibleIndex <= maxDecadeIndex - (maxDisplayedItems - pHost.Panel.Rows - 1));
+	}
+
 #if __WASM__
 	[TestMethod]
 	[Ignore("Fails on Fluent styles #17272")]

src/Uno.UI/UI/Xaml/Controls/CalendarView/Primitives/CalendarPanel.ModernCollectionBasePanel.cs

@@ -432,7 +432,21 @@ internal void ScrollItemIntoView(int index, ScrollIntoViewAlignment alignment, d
 				// then it will request GetContainerFromIndex and tries to focus it.
 				// So here we prepare the _effectiveViewport (which will most probably be re-updated by the ChangeView below),
 				// and then force a base_Measure()
-				_effectiveViewport.Y += newOffset - currentOffset;
+
+#if HAS_UNO
+				// Scrolling to the MaxDate and switching between Decade/Year/Month views keeps increasing the _effectiveViewport.Y
+				// even though there's no more items to show after MaxDate, causing empty rows in the viewport
+				if (newOffset >= sv.ScrollableHeight)
+				{
+					_effectiveViewport.Y = currentOffset;
+				}
+				else
+				{
+					_effectiveViewport.Y += newOffset - currentOffset;
+				}
+#else
+				_effectiveViewport.Y += newOffset - currentOffset;			
+#endif
 
 				sv.ChangeView(
 					horizontalOffset: null,