ModEntry.cs

using System;
// using Microsoft.Xna.Framework;
using StardewModdingAPI;
using StardewModdingAPI.Events;
using StardewValley;
using System.Reflection;
using StardewValley.Menus;
using System.Collections.Generic;

namespace KeyboardPP
{
    /// <summary>Improve keyboard support in the game.</summary>
    public class ModEntry : Mod {
        private FieldInfo SInputStateField;
        private MethodInfo OverrideButtonMethod;
        private IClickableMenu ActiveMenu;

        private Dictionary<SButton, SButton> titleKeyMap = new Dictionary<SButton, SButton>{
            {SButton.Left, SButton.A},
            {SButton.Right, SButton.D},
            {SButton.Up, SButton.W},
            {SButton.Down, SButton.S},
            {SButton.Space, SButton.MouseLeft},
            {SButton.Enter, SButton.MouseLeft},
        }; 

        public override void Entry(IModHelper helper) {
            Type type = helper.Input.GetType();
            SInputStateField = type.GetField("InputState", BindingFlags.NonPublic | BindingFlags.Instance);
            OverrideButtonMethod = SInputStateField.GetValue(helper.Input).GetType().GetMethod("OverrideButton");

            Helper.Events.Display.MenuChanged += this.OnEnterMenu;
            helper.Events.Input.ButtonPressed += this.OnButtonPressed;
        }

        private void TranslateEvent(SButton from, SButton to, bool down) {
            object inputState = SInputStateField.GetValue(Helper.Input);
            OverrideButtonMethod.Invoke(inputState, new object[] { from, false });
            OverrideButtonMethod.Invoke(inputState, new object[] { to, down });
        }

        private void OnEnterMenu(object sender, MenuChangedEventArgs e) {
            ActiveMenu = e.NewMenu;
        }

        private void OnButtonPressed(object sender, ButtonPressedEventArgs e) {
            bool isDown = e.IsDown(e.Button);

            if (!Context.IsWorldReady) {
                if (!titleKeyMap.ContainsKey(e.Button)) {
                    return;
                }

                TranslateEvent(e.Button, titleKeyMap[e.Button], isDown);
            }

            if(ActiveMenu == null) {
                return;
            }

            if (e.Button.IsUseToolButton() && e.Button != SButton.MouseLeft) {
                TranslateEvent(e.Button, SButton.MouseLeft, isDown);
            } else if (e.Button.IsActionButton() && e.Button != SButton.MouseRight) {
                TranslateEvent(e.Button, SButton.MouseRight, isDown);
            }

            // print button presses to the console window
            this.Monitor.Log($"{Game1.player.Name} pressed {e.Button}.", LogLevel.Debug);
        }
    }
}