Scroll ScrollViewer of ListBox by mouse wheel.

DependencyObjectExtensions.cs

using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using System.Windows;
using System.Windows.Media;

public static class DependencyObjectExtensions
{
    public static bool TryFindDescendant<T>(this DependencyObject reference, [NotNullWhen(true)] out T? descendant) where T : DependencyObject
    {
        var queue = new Queue<DependencyObject>();
        var parent = reference;

        while (parent is not null)
        {
            int count = VisualTreeHelper.GetChildrenCount(parent);
            for (int i = 0; i < count; i++)
            {
                var child = VisualTreeHelper.GetChild(parent, i);
                if (child is T buffer)
                {
                    descendant = buffer;
                    return true;
                }
                queue.Enqueue(child);
            }

            parent = (0 < queue.Count) ? queue.Dequeue() : null;
        }

        descendant = default;
        return false;
    }
}

ListBoxHelper.cs

using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;

internal static class ListBoxHelper
{
    public static int GetHorizontalScrollDivider(DependencyObject obj)
    {
        return (int)obj.GetValue(HorizontalScrollDividerProperty);
    }
    public static void SetHorizontalScrollDivider(DependencyObject obj, int value)
    {
        obj.SetValue(HorizontalScrollDividerProperty, value);
    }
    public static readonly DependencyProperty HorizontalScrollDividerProperty =
        DependencyProperty.RegisterAttached(
            "HorizontalScrollDivider",
            typeof(int),
            typeof(ListBoxHelper),
            new PropertyMetadata(
                defaultValue: 0,
                propertyChangedCallback: (d, e) =>
                {
                    if (d is not ListBox listBox)
                        return;

                    switch ((int)e.OldValue, (int)e.NewValue)
                    {
                        case (0, not 0):
                            listBox.PreviewMouseWheel += OnPreviewMouseWheel;
                            break;
                        case (_, 0):
                            listBox.PreviewMouseWheel -= OnPreviewMouseWheel;
                            break;
                    }
                }));

    private static void OnPreviewMouseWheel(object sender, MouseWheelEventArgs e)
    {
        var listBox = (ListBox)sender;
        if (!listBox.TryFindDescendant(out ScrollViewer? viewer))
            return;

        int divider = GetHorizontalScrollDivider(listBox);

        double offset = viewer.HorizontalOffset;
        offset += e.Delta / (double)divider;

        viewer.ScrollToHorizontalOffset(offset);
    }
}