Emulate multiple display input using GameViews

Display.RelativeMouseAt.cs

Vector3 RelativeMouseAt()
{
	var mouseOverWindow = EditorWindow.mouseOverWindow;
	System.Reflection.Assembly assembly = typeof(UnityEditor.EditorWindow).Assembly;
	Type type = assembly.GetType("UnityEditor.PlayModeView");
 
	int displayID = 0;
	if (type.IsInstanceOfType(mouseOverWindow))
	{
		var displayField = type.GetField("m_TargetDisplay", BindingFlags.NonPublic | BindingFlags.Instance);
		displayID = (int)displayField.GetValue(mouseOverWindow);
	}
 
	var pos = Input.mousePosition;
	pos[2] = displayID;
	return pos;
}