Let me mention a recent question:
math.stackexchange.com/questions/4119983. Two highly rated users felt the need to give full answers. But what's wrong with providing only a hint: "show that P is a projection" and let the OP to ask for more details if they feel the need? An answerer to the other question told me that he has no time to visit M.SE many times a day in order to start a dialogue with the OP, and thus he prefers to post full answers. (This is indeed a great excuse!)