From 5e8bce587b0abe31a218ebe2982ddbb5cb51bc35 Mon Sep 17 00:00:00 2001
From: David Oliver <david.oliver@nventive.com>
Date: Fri, 15 Oct 2021 11:28:09 -0400
Subject: [PATCH] fix(listview): [Android] Correct scroll offsets if necessary
 when scrolling upwards in list

---
 .../VirtualizingPanelLayout.Android.cs        | 26 ++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/src/Uno.UI/UI/Xaml/Controls/ListViewBase/VirtualizingPanelLayout.Android.cs b/src/Uno.UI/UI/Xaml/Controls/ListViewBase/VirtualizingPanelLayout.Android.cs
index e2851b261b0f..fc315a610290 100644
--- a/src/Uno.UI/UI/Xaml/Controls/ListViewBase/VirtualizingPanelLayout.Android.cs
+++ b/src/Uno.UI/UI/Xaml/Controls/ListViewBase/VirtualizingPanelLayout.Android.cs
@@ -724,6 +724,8 @@ private int ComputeScrollRange(RecyclerView.State state, Orientation orientation
 				remainingGroupExtent = remainingGroups * lastGroup.HeaderExtent;
 			}
 
+			CorrectForEstimationErrors();
+
 			var range = ContentOffset + remainingItemExtent + remainingGroupExtent + headerExtent + footerExtent +
 				//TODO: An inline group header might actually be the view at the bottom of the viewport, we should take this into account
 				GetChildEndWithMargin(base.GetChildAt(FirstItemView + ItemViewCount - 1));
@@ -732,6 +734,26 @@ private int ComputeScrollRange(RecyclerView.State state, Orientation orientation
 			return range;
 		}
 
+		/// <summary>
+		/// Correct the scroll offset, eg if items were added/removed or had their databound heights changed while they were scrolled out
+		/// of view.
+		/// </summary>
+		private void CorrectForEstimationErrors()
+		{
+			if (ContentOffset < 0)
+			{
+				// Scroll offset should always be non-negative
+				ContentOffset = 0;
+			}
+
+			var firstVisible = GetFirstVisibleIndexPath();
+			if (firstVisible.Row == 0 && firstVisible.Section == 0)
+			{
+				// If first item is in view, we can set ContentOffset exactly
+				ContentOffset = -GetContentStart();
+			}
+		}
+
 		/// <summary>
 		/// Update the internal state of the layout, as well as 'floating' views like group headers, when the scrolled offset changes.
 		/// </summary>
@@ -742,7 +764,9 @@ private int ComputeScrollRange(RecyclerView.State state, Orientation orientation
 		private void ApplyOffset(int delta)
 		{
 			ContentOffset -= delta;
-			Debug.Assert(ContentOffset >= 0, "ContentOffset must be non-negative.");
+
+			CorrectForEstimationErrors();
+
 			foreach (var group in _groups)
 			{
 				group.Start += delta;