From the Manual:
[quote:2rnzlpsa]If an individual layer has been scrolled, scaled or rotated, these expressions do not take that in to account - for that case, use the layer versions below.
Return the position of the mouse cursor in game co-ordinates, with scrolling, scaling and rotation taken in to account for the given layer. The layer can be identified either by a string of its name or its zero-based index (e.g. Mouse.X(0)).