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 @@ internal void EnsureVisibleEdgedColumnsWidth()
  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 @@ internal void EnsureVisibleEdgedColumnsWidth()
  }
  }
 
- 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 @@ internal DataGridRow OwningRow
  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 @@ internal DataGrid OwningGrid
  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;
  }