This is something that SOOOOOO irks me. The question is not about keeping the shades open during the day, and closed during the night. That is not the point. At night, it does not matter.

The problem is glare. During the day, the person at the window is generally not bothered by the glare that comes in the window and shines on everything else in the cabin, especially when that window is the ONLY window open, or when the sun is at a low angle in the sky.

On an MRU-LHR flight last year, seats got screwed so my partner and I ended up in 4EF instead of 4K/5K on a 744. Turns out the Female Passenger in 4A insisted her window stay open (only one), despite the fact that glare interferred with our screens. After the first polite request & refusal, then a 2nd involving the purser, then a 3rd escalated to the captain (who said the pax could do what she wanted - it's her shade), no resolution.

So, at an appropriate point later in the flight, I was able to tilt my screen (in direct sunlight) to reflect directly back in her face, which I kept directly spot-on her face as long as I could. Her resulting horror when I persisted with this for about 10 minutes was priceless.

This is a no-win situation when people are so selfish as to ignore the reasonable requests from other passengers - even long haul First is no sanctuary from such inconsideration.