From f632a3e95db9849941e13285dbea3ba9dfc580e4 Mon Sep 17 00:00:00 2001
From: Matthew Rajala <matthew.rajala@polymtl.ca>
Date: Mon, 25 Nov 2024 13:18:20 -0500
Subject: [PATCH 1/2] chore: scrollviewer offset solution

---
 .../Given_CalendarView.cs                     | 38 +++++++++++++++++++
 ...CalendarPanel.ModernCollectionBasePanel.cs | 10 ++++-
 2 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_CalendarView.cs b/src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_CalendarView.cs
index 851a539b1ddd..4b29f020d760 100644
--- a/src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_CalendarView.cs
+++ b/src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Controls/Given_CalendarView.cs
@@ -24,6 +24,8 @@ namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Controls;
 [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 @@ public async Task When_MinDate_Has_Different_Offset()
 		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")]
diff --git a/src/Uno.UI/UI/Xaml/Controls/CalendarView/Primitives/CalendarPanel.ModernCollectionBasePanel.cs b/src/Uno.UI/UI/Xaml/Controls/CalendarView/Primitives/CalendarPanel.ModernCollectionBasePanel.cs
index 7cd5d8476da8..1fe4bc3120e5 100644
--- a/src/Uno.UI/UI/Xaml/Controls/CalendarView/Primitives/CalendarPanel.ModernCollectionBasePanel.cs
+++ b/src/Uno.UI/UI/Xaml/Controls/CalendarView/Primitives/CalendarPanel.ModernCollectionBasePanel.cs
@@ -432,7 +432,15 @@ 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 (newOffset >= sv.ScrollableHeight)
+				{
+					_effectiveViewport.Y = currentOffset;
+				}
+				else
+				{
+					_effectiveViewport.Y += newOffset - currentOffset;
+				}
 
 				sv.ChangeView(
 					horizontalOffset: null,

From 16ca3fbf4aa8ad28f80a459ce49c14f6cae8778e Mon Sep 17 00:00:00 2001
From: Matthew Rajala <matthew.rajala@polymtl.ca>
Date: Thu, 5 Dec 2024 10:42:41 -0500
Subject: [PATCH 2/2] chore: add issue mention

---
 .../Primitives/CalendarPanel.ModernCollectionBasePanel.cs   | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/Uno.UI/UI/Xaml/Controls/CalendarView/Primitives/CalendarPanel.ModernCollectionBasePanel.cs b/src/Uno.UI/UI/Xaml/Controls/CalendarView/Primitives/CalendarPanel.ModernCollectionBasePanel.cs
index 1fe4bc3120e5..7befaaec8e9f 100644
--- a/src/Uno.UI/UI/Xaml/Controls/CalendarView/Primitives/CalendarPanel.ModernCollectionBasePanel.cs
+++ b/src/Uno.UI/UI/Xaml/Controls/CalendarView/Primitives/CalendarPanel.ModernCollectionBasePanel.cs
@@ -433,6 +433,9 @@ internal void ScrollItemIntoView(int index, ScrollIntoViewAlignment alignment, d
 				// So here we prepare the _effectiveViewport (which will most probably be re-updated by the ChangeView below),
 				// and then force a base_Measure()
 
+#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;
@@ -441,6 +444,9 @@ internal void ScrollItemIntoView(int index, ScrollIntoViewAlignment alignment, d
 				{
 					_effectiveViewport.Y += newOffset - currentOffset;
 				}
+#else
+				_effectiveViewport.Y += newOffset - currentOffset;			
+#endif
 
 				sv.ChangeView(
 					horizontalOffset: null,