Unity input system interaction to detect if opposing keys like left/right is pressed simultaneously and delay the interaction to see if one of the key's is released. One use cases is, if using a state machine with Input magnitude for character movement, character can keep moving from one direction to the opposite without going to 'Idle' state.

OpposingKeyPressInteraction.cs

using UnityEngine;
using UnityEngine.InputSystem;

#if UNITY_EDITOR
[UnityEditor.InitializeOnLoad]
#endif
public class OpposingKeyPressInteraction : IInputInteraction
{
    public float HoldDuration = 0.15f;

    private InputInteractionContext ctx;
    private float _timer;

#if UNITY_EDITOR
    static OpposingKeyPressInteraction()
    {
        Initialize();
    }
#endif

    [RuntimeInitializeOnLoadMethod]
    static void Initialize()
    {
        InputSystem.RegisterInteraction<OpposingKeyPressInteraction>();
    }
    
    public void Process(ref InputInteractionContext context)
    {
        ctx = context;

        if (ctx.control.IsPressed() && context.ReadValue<Vector2>().magnitude == 0)
        {
            _timer = HoldDuration;
            InputSystem.onAfterUpdate -= OnUpdate; // Safeguard for duplicate registrations
            InputSystem.onAfterUpdate += OnUpdate;
            return;
        }

        if (ctx.phase == InputActionPhase.Waiting)
        {
            context.PerformedAndStayStarted();
            return;
        }

        if (ctx.phase == InputActionPhase.Started)
        {
            context.Performed();
            return;
        }
    }

    private void OnUpdate()
    {
        if (_timer > 0)
        {
            _timer -= Time.deltaTime;

            if (ctx.ReadValue<Vector2>().magnitude != 0)
            {
                ctx.PerformedAndStayStarted();
                InputSystem.onAfterUpdate -= OnUpdate;
                _timer = 0;
            }
        }
        else
        {
            if (!ctx.ControlIsActuated())
            {
                ctx.Canceled();
            }

            InputSystem.onAfterUpdate -= OnUpdate;
        }
    }

    public void Reset()
    {
        InputSystem.onAfterUpdate -= OnUpdate;
        _timer = HoldDuration;

        if (!ctx.ControlIsActuated() && (ctx.phase == InputActionPhase.Started || ctx.phase == InputActionPhase.Performed))
        {
            ctx.Canceled();
        }
    }
}