On the screen, Arthur was looking down on the people gathered to hear him sing live. This was the last scene of the movie in which Arthur would sing the song that he had just completed in front of an audience that would include Charlie and Claire.
It was the accumulation of all that Arthur had went through in the movie and how he had matured as a person.
'His eyes are a bit different.'
Alexander who has his sights fixed on the screen thought.
Throughout the movie, the emotions in Arthur's eyes were pretty subtle and hidden, like he didn't want others to know what he was feeling. He was a very reserved character and his eyes portrayed that very well.
But in the last scene, there was a brightness in his eyes as Arthur started his performance, like he had finally found the hope he was looking for.