Answer? Nothing. Nothing but a new shower custom to the space. Paint the walls white, and move on until ready to re do the KLUDGE bath.

Seems like the shower might have run into the window if they extended it and the previous owners probably didn't want to pay to upgrade it so they took the cheap route. There's no fix other than a gut, but painting the walls white would lessen the effect.

I don’t even know how you would get back there to physically paint it? Uggh

Its a prefabbed shower and unfortunately that's where it ends. You have to look at what the past owner was facing with that window in the way. They probably paid a 1/3 of what it would have cost to move the window and extend the shower to that wall. That would involve a on site fabricated shower beyond 5' which would need a non prefabbed pan and shower enclosure.

You "get back" There with a eeny roller on a pole : ) Above it? Get in the shower. Turn off the water lol. A pain, but WHITE would help. A lot. Lower the contrast. In fact, if you get in the shower on a stool? You can roll half way down that narrow spot, pretty easily. The remainder as stated

I would try to match that white and do the whole bath. Add color with towels, bathmats, new shades and accessories.