Repository: AvaloniaUI/Avalonia
File: src/Avalonia.Controls/ComboBox.cs (C#)

Code before:
    /// <summary>
    /// A drop-down list control.
    /// </summary>
    [TemplatePart("PART_Popup", typeof(Popup), IsRequired = true)]
    [PseudoClasses(pcDropdownOpen, pcPressed)]
    public class ComboBox : SelectingItemsControl
    {
        internal const string pcDropdownOpen = ":dropdownopen";
        internal const string pcPressed = ":pressed";

        /// <summary>
        /// The default value for the <see cref="ItemsControl.ItemsPanel"/> property.
        /// </summary>
        private static readonly FuncTemplate<Panel?> DefaultPanel =
            new(() => new VirtualizingStackPanel());

        /// <summary>
        /// Defines the <see cref="IsDropDownOpen"/> property.
        /// </summary>
        public static readonly StyledProperty<bool> IsDropDownOpenProperty =
            AvaloniaProperty.Register<ComboBox, bool>(nameof(IsDropDownOpen));

        /// <summary>
        /// Defines the <see cref="IsEditable"/> property.
        /// </summary>
        public static readonly DirectProperty<ComboBox, bool> IsEditableProperty =
            AvaloniaProperty.RegisterDirect<ComboBox, bool>(nameof(IsEditable),
                o => o.IsEditable,
                (o, v) => o.IsEditable = v);

        /// <summary>
        /// Defines the <see cref="MaxDropDownHeight"/> property.
        /// </summary>
        public static readonly StyledProperty<double> MaxDropDownHeightProperty =
            AvaloniaProperty.Register<ComboBox, double>(nameof(MaxDropDownHeight), 200);

        /// <summary>
        /// Defines the <see cref="SelectionBoxItem"/> property.
        /// </summary>
        public static readonly DirectProperty<ComboBox, object?> SelectionBoxItemProperty =
            AvaloniaProperty.RegisterDirect<ComboBox, object?>(nameof(SelectionBoxItem), o => o.SelectionBoxItem);

        /// <summary>
        /// Defines the <see cref="PlaceholderText"/> property.
        /// </summary>
        public static readonly StyledProperty<string?> PlaceholderTextProperty =
            AvaloniaProperty.Register<ComboBox, string?>(nameof(PlaceholderText));

        /// <summary>
        /// Defines the <see cref="PlaceholderForeground"/> property.
        /// </summary>

Review comment (suggestion):
Focusing the `ComboBox` when `IsEditable = true` should auto-select the text. (It matches WPF, Win32, and Avalonia's `AutoCompleteBox`).

Code after:
    /// <summary>
    /// A drop-down list control.
    /// </summary>
    [TemplatePart("PART_Popup", typeof(Popup), IsRequired = true)]
    [TemplatePart("PART_EditableTextBox", typeof(TextBox), IsRequired = false)]
    [PseudoClasses(pcDropdownOpen, pcPressed)]
    public class ComboBox : SelectingItemsControl
    {
        internal const string pcDropdownOpen = ":dropdownopen";
        internal const string pcPressed = ":pressed";

        /// <summary>
        /// The default value for the <see cref="ItemsControl.ItemsPanel"/> property.
        /// </summary>
        private static readonly FuncTemplate<Panel?> DefaultPanel =
            new(() => new VirtualizingStackPanel());

        /// <summary>
        /// Defines the <see cref="IsDropDownOpen"/> property.
        /// </summary>
        public static readonly StyledProperty<bool> IsDropDownOpenProperty =
            AvaloniaProperty.Register<ComboBox, bool>(nameof(IsDropDownOpen));

        /// <summary>
        /// Defines the <see cref="IsEditable"/> property.
        /// </summary>
        public static readonly StyledProperty<bool> IsEditableProperty =
            AvaloniaProperty.Register<ComboBox, bool>(nameof(IsEditable));

        /// <summary>
        /// Defines the <see cref="MaxDropDownHeight"/> property.
        /// </summary>
        public static readonly StyledProperty<double> MaxDropDownHeightProperty =
            AvaloniaProperty.Register<ComboBox, double>(nameof(MaxDropDownHeight), 200);

        /// <summary>
        /// Defines the <see cref="SelectionBoxItem"/> property.
        /// </summary>
        public static readonly DirectProperty<ComboBox, object?> SelectionBoxItemProperty =
            AvaloniaProperty.RegisterDirect<ComboBox, object?>(nameof(SelectionBoxItem), o => o.SelectionBoxItem);

        /// <summary>
        /// Defines the <see cref="PlaceholderText"/> property.
        /// </summary>
        public static readonly StyledProperty<string?> PlaceholderTextProperty =
            AvaloniaProperty.Register<ComboBox, string?>(nameof(PlaceholderText));

        /// <summary>
        /// Defines the <see cref="PlaceholderForeground"/> property.
        /// </summary>
        public static readonly StyledProperty<IBrush?> PlaceholderForegroundProperty =