From 362745a55ec7df25a0f4060c47ecb4d89b11ae6e Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Sat, 21 May 2016 16:53:42 +0200
Subject: [PATCH] Fix scrolling algorithm

---
 .../Presenters/ThingamybobPresenter.cs             | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/src/Avalonia.Controls/Presenters/ThingamybobPresenter.cs b/src/Avalonia.Controls/Presenters/ThingamybobPresenter.cs
index 73fd0c8cfa..00f0c92535 100644
--- a/src/Avalonia.Controls/Presenters/ThingamybobPresenter.cs
+++ b/src/Avalonia.Controls/Presenters/ThingamybobPresenter.cs
@@ -31,20 +31,26 @@ namespace Avalonia.Controls.Presenters
         {
             get
             {
-                return new Vector(0, _firstIndex * AverageItemSize);
+                return new Vector(0, (_firstIndex * AverageItemSize) + (_panel?.PixelOffset ?? 0));
             }
 
             set
             {
                 var count = _lastIndex - _firstIndex;
-                _firstIndex = (int)(value.Y / AverageItemSize);
+                var firstIndex = (int)(value.Y / AverageItemSize);
+                var firstIndexChanged = _firstIndex != firstIndex;
+                _firstIndex = firstIndex;
                 _lastIndex = _firstIndex + count;
                 _panel.PixelOffset = value.Y % AverageItemSize;
-                Renumber();
+
+                if (firstIndexChanged)
+                {
+                    Renumber();
+                }
             }
         }
 
-        Size IScrollable.Viewport => new Size(1, (_lastIndex - _firstIndex) * AverageItemSize);
+        Size IScrollable.Viewport => new Size(1, _panel?.Bounds.Height ?? 0);
         Size IScrollable.ScrollSize => new Size(0, 1);
         Size IScrollable.PageScrollSize => new Size(0, 1);