Expander control is not narrated

[zlatanov]

• .NET Core Version: 3.0.100
• Windows version: 1903
• Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes

The problem has already been described here: #1015. What I don't understand in the solution there is why though the expander is exposed only when not in ItemsHost Panel.

wpf/src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Automation/Peers/GroupItemAutomationPeer.cs

Lines 148 to 161 in 0166abb

	Panel itemsHost = owner.ItemsHost;
	if (itemsHost == null)
	{
	if (_expanderPeer == null)
	{
	return null;
	}
	else
	{
	children.Add(_expanderPeer);
	return children;
	}
	}

If I have a DataGrid with grouping, the headers of the groups are left out from the UI Automation tree and thus are practically inaccessible.