mu3-mods/SortByInternalDifficulty/MU3/patch_UISortButton.cs

using MU3.CustomUI;
using System.Reflection;
using UnityEngine;

namespace MU3;

class patch_UISortButton: UISortButton {
    private MU3UIImageChanger _selectPreviewImageChanger;

    protected override void Awake() {
        base.Awake();

        var spritesFi = typeof(MU3UIImageChanger).GetField("_sprites", BindingFlags.Instance | BindingFlags.NonPublic);
        var changer = _selectPreviewImageChanger;

        var oldSprites = (Sprite[])spritesFi.GetValue(changer);
        var n = oldSprites.Length;
        var newSprites = new Sprite[n + 1];

        for(int i = 0; i < n; ++i) {
            newSprites[i] = oldSprites[i];
        }

        Texture2D tex = new Texture2D(2, 2);
        tex.LoadImage(InternalSortPreview.Bytes);

        newSprites[n] = Sprite.Create(tex, new Rect(0, 0, 254, 121), newSprites[0].pivot);

        spritesFi.SetValue(changer, newSprites);
    }
}