Manners among theatergoers have been neglected, such that probably most offenders do not even know they are being inconsiderate.
The truly rude will always do as they like, but I believe there is a large group of perfectly polite, but unaware, theatergoers who would welcome enlightenment.: You are not wrong about its being inconsiderate to block another person’s view, and an apologetic whisper might help if it is only the row directly in front of you.
Miss Manners often says that it is rude to correct another person’s manners. How much worse to scold someone who has not yet done anything wrong -- an adult, in fact, who has gone to some trouble in the hopes of enjoying an evening out! She said, “Oh, geez, you don’t need to wear a mask.” She acted as though I was being ridiculous and also as though she was offended.