Fixes scrolling bug in grouped DataGrids

Fixes #5

samples/DataGridSample/DataGridSample.csproj

@@ -12,8 +12,8 @@
     </EmbeddedResource>
   </ItemGroup>
   <ItemGroup>
-    <PackageReference Include="Avalonia" Version="0.6.2-build5521-beta" />
+    <PackageReference Include="Avalonia" Version="0.6.2-build5800-beta" />
-    <PackageReference Include="Avalonia.Desktop" Version="0.6.2-build5521-beta" />
+    <PackageReference Include="Avalonia.Desktop" Version="0.6.2-build5800-beta" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\src\Avalonia.DataGrid.Themes.Default\Avalonia.DataGrid.Themes.Default.csproj" />

src/Avalonia.DataGrid.Themes.Default/Avalonia.DataGrid.Themes.Default.csproj

@@ -5,7 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Avalonia" Version="0.6.2-build5521-beta" />
+    <PackageReference Include="Avalonia" Version="0.6.2-build5800-beta" />
   </ItemGroup>
   <ItemGroup>
     <Compile Update="**\*.xaml.cs">

src/Avalonia.DataGrid/Avalonia.DataGrid.csproj

@@ -5,7 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Avalonia" Version="0.6.2-build5521-beta" />
+    <PackageReference Include="Avalonia" Version="0.6.2-build5800-beta" />
   </ItemGroup>
 
 </Project>

src/Avalonia.DataGrid/DataGrid.cs

@@ -1512,7 +1512,7 @@ namespace Avalonia.Controls
         {
             get
             {
-                return RowsPresenterAvailableSize?.Height ?? 0;
+                return RowsPresenterEstimatedAvailableHeight ?? 0;
             }
         }
 
@@ -1800,7 +1800,11 @@ namespace Avalonia.Controls
                 _rowsPresenterAvailableSize = value;
             }
         }
-
+        internal double? RowsPresenterEstimatedAvailableHeight
+        {
+            get;
+            set;
+        }
 
         internal double[] RowGroupSublevelIndents
         {

src/Avalonia.DataGrid/DataGridRowGroupHeader.cs

@@ -468,7 +468,7 @@ namespace Avalonia.Controls
                     // Do these even if the OwningGrid is null in case it could improve the Designer experience for a standalone DataGridRowGroupHeader
                     RowGroupInfo.IsVisible = isVisible;
                 }
-                else
+                else if(RowGroupInfo.IsVisible != isVisible)
                 {
                     OwningGrid.OnRowGroupHeaderToggled(this, isVisible, setCurrent);
                 }

src/Avalonia.DataGrid/Primitives/DataGridRowsPresenter.cs

@@ -20,7 +20,20 @@ namespace Avalonia.Controls.Primitives
             get;
             set;
         }
-        
+
+        private double _measureHeightOffset = 0;
+        private double CalculateEstimatedAvailableHeight(Size availableSize)
+        {
+            if(!Double.IsPositiveInfinity(availableSize.Height))
+            {
+                return availableSize.Height + _measureHeightOffset;
+            }
+            else
+            {
+                return availableSize.Height;
+            }
+        }
+
         /// <summary>
         /// Arranges the content of the <see cref="T:Avalonia.Controls.Primitives.DataGridRowsPresenter" />.
         /// </summary>
@@ -37,6 +50,16 @@ namespace Avalonia.Controls.Primitives
                 return base.ArrangeOverride(finalSize);
             }
 
+            if(OwningGrid.RowsPresenterAvailableSize.HasValue)
+            {
+                var availableHeight = OwningGrid.RowsPresenterAvailableSize.Value.Height;
+                if(!Double.IsPositiveInfinity(availableHeight))
+                {
+                    _measureHeightOffset = finalSize.Height - availableHeight;
+                    OwningGrid.RowsPresenterEstimatedAvailableHeight = finalSize.Height;
+                }
+            }
+
             OwningGrid.OnFillerColumnWidthNeeded(finalSize.Width);
 
             double rowDesiredWidth = OwningGrid.ColumnsInternal.VisibleEdgedColumnsWidth + OwningGrid.ColumnsInternal.FillerColumn.FillerWidth;
@@ -97,6 +120,7 @@ namespace Avalonia.Controls.Primitives
             // The DataGrid uses the RowsPresenter available size in order to autogrow
             // and calculate the scrollbars
             OwningGrid.RowsPresenterAvailableSize = availableSize;
+            OwningGrid.RowsPresenterEstimatedAvailableHeight = CalculateEstimatedAvailableHeight(availableSize);
 
             OwningGrid.OnRowsMeasure();