Fix DataGridColumn.DisplayIndex initial value handling

samples/ControlCatalog/Pages/DataGridPage.xaml

@@ -38,8 +38,8 @@
           <DataGrid.Columns>
             <DataGridTextColumn Header="Country" Binding="{Binding Name}" Width="6*" />
             <DataGridTextColumn Header="Region" Binding="{Binding Region}" Width="4*" />
-            <DataGridTextColumn Header="Population" Binding="{Binding Population}" Width="3*" />
-            <DataGridTextColumn Header="Area" Binding="{Binding Area}" Width="3*" />
+            <DataGridTextColumn DisplayIndex="3" Header="Population" Binding="{Binding Population}" Width="3*" />
+            <DataGridTextColumn DisplayIndex="2" Header="Area" Binding="{Binding Area}" Width="3*" />
             <DataGridTextColumn Header="GDP" Binding="{Binding GDP}" Width="3*" />
           </DataGrid.Columns>
         </DataGrid>

src/Avalonia.Controls.DataGrid/DataGrid.cs

@@ -2285,6 +2285,17 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <summary>
+        /// Comparator class so we can sort list by the display index
+        /// </summary>
+        public class DisplayIndexComparer : IComparer<DataGridColumn>
+        {
+            int IComparer<DataGridColumn>.Compare(DataGridColumn x, DataGridColumn y)
+            {
+                return (x.DisplayIndexWithFiller < y.DisplayIndexWithFiller) ? -1 : 1;
+            }
+        }
+
         /// <summary>
         /// Builds the visual tree for the column header when a new template is applied.
         /// </summary>
@@ -2309,8 +2320,11 @@ namespace Avalonia.Controls
                     ColumnsInternal.FillerColumn.IsRepresented = false;
                 }
                 _columnHeadersPresenter.OwningGrid = this;
-                // Columns were added before before our Template was applied, add the ColumnHeaders now
-                foreach (DataGridColumn column in ColumnsItemsInternal)
+
+                // Columns were added before our Template was applied, add the ColumnHeaders now
+                List<DataGridColumn> sortedInternal = new List<DataGridColumn>(ColumnsItemsInternal);
+                sortedInternal.Sort(new DisplayIndexComparer());
+                foreach (DataGridColumn column in sortedInternal)
                 {
                     InsertDisplayedColumnHeader(column);
                 }