no, you have to check the position of the mouse pointer to do this.
like
Code:
function show_object()
{
if((pointer.x >= object.x) && (pointer.y >= object.x) && (pointer.x <= object.x + object.width) && (pointer.y <= object.y + object.height))
{
object.visible = on;
}
}
on_mouse_left = show_object;
EDIT: This works for panels, for entities you have to convert the pointer coordinates to 3D.
not tested