Does masturbation invalidate I'tikaf? And is it obligatory to make up for a spoiled I'tikaf? And what is the indication that shifted the Prophet's — may Allah's prayers and peace be upon him — making up for his I'tikaf from obligation to recommendation?

A wet dream by a person in i'tikaf does not affect the validity of their i'tikaf. However, masturbation, according to scholars, invalidates i'tikaf. Madhy (pre-seminal fluid) does not invalidate i'tikaf unless it results from direct physical contact with desire, according to some scholars. As for wady (a thick white discharge that sometimes comes out after urination), it does not invalidate i'tikaf. Making up a missed i'tikaf is obligatory if it was vowed. Scholars differ regarding making up a voluntary i'tikaf if it was invalidated, and the preponderant opinion is that it is not obligatory to make it up. As for making up i'tikaf for someone who missed it due to an excuse, there is a difference of opinion, and the preponderant view is that making it up is not legislated because the Prophet (peace be upon him) did not legislate it, but only intended it. However, making up a sunnah i'tikaf for someone who invalidated it after commencing it is more preferable and better.