InputModuleWithLockedCursor.cs

using UnityEngine;
using UnityEngine.EventSystems;

namespace SceneTransition.Demo
{
    /// <summary>
    /// Custom Input module:
    ///    If cursor is locked, send mouse events from center of the screen.
    /// 
    /// Found this hack here https://forum.unity.com/threads/fake-mouse-position-in-4-6-ui-answered.283748/#post-3600421
    /// </summary>
    public class InputModuleWithLockedCursor : StandaloneInputModule
    {
        protected override MouseState GetMousePointerEventData(int id)
        {
            var mouseState = base.GetMousePointerEventData(id);
            if (Cursor.lockState == CursorLockMode.Locked)
            {
                var buttonData = mouseState.GetButtonState(0).eventData.buttonData;
                
                buttonData.position = new Vector2(Screen.width / 2.0f, Screen.height / 2.0f);
                buttonData.delta = Vector2.zero;
                eventSystem.RaycastAll(buttonData, m_RaycastResultCache);
                RaycastResult firstRayCast = FindFirstRaycast(m_RaycastResultCache);
                buttonData.pointerCurrentRaycast = firstRayCast;                
                mouseState.SetButtonState(PointerEventData.InputButton.Left, StateForMouseButton(0), buttonData);
            }

            return mouseState;
        }
    }
}