GSS2Rework/GSS2.UI.Core/Behaviors/GridColumnHideBehavior.cs

// Оригинальное решение для проблемы сворачивания колонок в Grid от
// https://github.com/AvaloniaUI/Avalonia/discussions/6773#discussioncomment-1514604

using Avalonia;
using Avalonia.Controls;

namespace GSS2.UI.Core.Behaviors;

public static class GridColumnHideBehavior
{
    private readonly static GridLength ZeroWidth = new GridLength(0, GridUnitType.Pixel);

    private readonly static AttachedProperty<GridLength?> LastWidthProperty =
        AvaloniaProperty.RegisterAttached<ColumnDefinition, GridLength?>("LastWidth", typeof(GridColumnHideBehavior), default);

    public readonly static AttachedProperty<bool> IsVisibleProperty =
         AvaloniaProperty.RegisterAttached<ColumnDefinition, bool>("IsVisible", typeof(GridColumnHideBehavior), true,
            coerce: (element, visibility) =>
            {

                var lastWidth = element.GetValue(LastWidthProperty);
                if (visibility && lastWidth is { })
                    element.SetValue(ColumnDefinition.WidthProperty, lastWidth);
                else if (!visibility)
                {
                    element.SetValue(LastWidthProperty, element.GetValue(ColumnDefinition.WidthProperty));
                    element.SetValue(ColumnDefinition.WidthProperty, ZeroWidth);
                }
                return visibility;
            }
        );

    public static bool GetIsVisible(ColumnDefinition columnDefinition)
    {
        return columnDefinition.GetValue(IsVisibleProperty);
    }

    public static void SetIsVisible(ColumnDefinition columnDefinition, bool visibility)
    {
        columnDefinition.SetValue(IsVisibleProperty, visibility);
    }
}