I say, and with God's success: The pillar of I'tikaf is staying; because it indicates that, even if one leaves for an hour without a valid excuse during a mandatory I'tikaf, their I'tikaf is invalidated; because leaving contradicts staying, and anything that contradicts something else is the same whether it is little or much: like eating and drinking during fasting, and being in a state of impurity during purity. Refer to: Tabyin al-Haqaiq 1: 351, and Al-Mabsut 3: 119, and God knows best.