Is it permissible to perform ablution (wudu') during ghusl (ritual bath) from major impurity (janabah) in the shower stall, knowing that standing on the running water that falls from the body during ghusl might nullify ablution?

Standing on water that has fallen from the body during ghusl (ritual bath) or wudu (ablution) does not invalidate wudu because this water is pure. Even if the falling water were impure, wudu would not be invalidated by it; rather, it is invalidated by impurities that exit the person performing ablution himself. If any dirt or impurities cling to one after purification, they should be removed by washing. The Sunnah indicates that the Prophet, peace and blessings be upon him, used to wash his feet in another place after ghusl if the first place was not clean, so that his feet would not become soiled.