Hello CSchmitt,

Thank you for the screenshots you have provided.

The blue highlight rectangle around the ComboEditorItemControl from your first image indicates the currently active/focused element (the one that will get selected if Enter is pressed) and not the selected one.

In order to get the underlying data item of the currently active combo item inside the PreviewKeyDown event of the XamComboEditor prior to the SelectionChanged event, I can suggest you go down through the visual tree of the editor's dropdown and get the respective ComboEditorItemControl whose focus rectangle is currently visible.

Code-behind:

private void combo_PreviewKeyDown(object sender, KeyEventArgs e)
{
    var combo = (sender as XamComboEditor);
            
    if (combo.IsDropDownOpen && e.Key == Key.Enter)
    {
        var item = GetActiveItem(combo);
        MessageBox.Show("Active item: " + item);
    }
}

private static object GetActiveItem(XamComboEditor combo)
{
    var dropdownPopup = GetOpenPopups().FirstOrDefault().Child;
    if (dropdownPopup == null)
        return null;
    var itemsPanel = Utilities.GetDescendantFromName(dropdownPopup, "ItemsPanel") as ItemsPanel;
    if (itemsPanel == null)
        return null;
    // Get the ComboEditorItemControl whose inner focus rectangle is visible.
    // (the blue highlight around the item indicates it is visible)
    //
    var activeComboEditorItemControl = itemsPanel.Children
                                            .OfType<ComboEditorItemControl>()
                                            .FirstOrDefault(it => (Utilities.GetDescendantFromName(it, "FocusVisualElement") as Rectangle).IsVisible);
            
    var item = activeComboEditorItemControl.Item.Data;
    return item;
}

I have attached a sample application that demonstrates the approach from above.

If you have any questions, please let me know.