Fix nth child selector in data grid header (#15482)

* Removed index to display index map so that nth child selector can work with current data.

* Iterate to find the index of the column taking visibility into account.

* Don't rely on VisibleColumnCount since it is updated later.

* Don't take visibility into account when evaluating nth-child and nth-last-child.

src/Avalonia.Controls.DataGrid/DataGridColumnCollection.cs

@@ -12,7 +12,6 @@ namespace Avalonia.Controls
 {
     internal class DataGridColumnCollection : ObservableCollection<DataGridColumn>
     {
-        private readonly Dictionary<int, int> _columnsMap = new Dictionary<int, int>();
         private readonly DataGrid _owningGrid;
 
         public DataGridColumnCollection(DataGrid owningGrid)
@@ -280,12 +279,10 @@ namespace Avalonia.Controls
             VisibleStarColumnCount = 0;
             VisibleEdgedColumnsWidth = 0;
             VisibleColumnCount = 0;
-            _columnsMap.Clear();
 
             for (int columnIndex = 0; columnIndex < ItemsInternal.Count; columnIndex++)
             {
                 var item = ItemsInternal[columnIndex];
-                _columnsMap[columnIndex] = item.DisplayIndex;
                 if (item.IsVisible)
                 {
                     VisibleColumnCount++;
@@ -299,11 +296,6 @@ namespace Avalonia.Controls
             }
         }
 
-        internal int GetColumnDisplayIndex(int columnIndex)
-        {
-            return _columnsMap.TryGetValue(columnIndex, out var displayIndex) ? displayIndex : -1;
-        }
-
         internal DataGridColumn GetColumnAtDisplayIndex(int displayIndex)
         {
             if (displayIndex < 0 || displayIndex >= ItemsInternal.Count || displayIndex >= DisplayIndexMap.Count)

src/Avalonia.Controls.DataGrid/Primitives/DataGridCellsPresenter.cs

@@ -53,13 +53,13 @@ namespace Avalonia.Controls.Primitives
         int IChildIndexProvider.GetChildIndex(ILogical child)
         {
             return child is DataGridCell cell
-                ? OwningGrid.ColumnsInternal.GetColumnDisplayIndex(cell.ColumnIndex)
+                ? cell.OwningColumn?.DisplayIndex ?? -1
                 : throw new InvalidOperationException("Invalid cell type");
         }
 
         bool IChildIndexProvider.TryGetTotalCount(out int count)
         {
-            count = OwningGrid.ColumnsInternal.VisibleColumnCount;
+            count = Children.Count - 1; // Adjust for filler column
             return true;
         }
 

src/Avalonia.Controls.DataGrid/Primitives/DataGridColumnHeadersPresenter.cs

@@ -118,13 +118,13 @@ namespace Avalonia.Controls.Primitives
         int IChildIndexProvider.GetChildIndex(ILogical child)
         {
             return child is DataGridColumnHeader header
-                ? OwningGrid.ColumnsInternal.GetColumnDisplayIndex(header.ColumnIndex)
+                ? header.OwningColumn?.DisplayIndex ?? -1
                 : throw new InvalidOperationException("Invalid cell type");
         }
 
         bool IChildIndexProvider.TryGetTotalCount(out int count)
         {
-            count = OwningGrid.ColumnsInternal.VisibleColumnCount;
+            count = Children.Count - 1; // Adjust for filler column
             return true;
         }