Sandbox++
Released
An open-source, community-driven successor to GMod's Sandbox. Work in progress!
Player/Player.Undo.cs
Partial Player class adding an Undo field that references the per-player undo stack from UndoSystem using the network owner's SteamId.
Networking
public sealed partial class Player
{
/// <summary>
/// Access the undo system for this player
/// </summary>
public UndoSystem.PlayerStack Undo => UndoSystem.Current.For( Network.Owner.SteamId );
}