Molding, a back splash (material of your choice), fill the gap with scribed wood and paint ot match the wall. It sounds like someone was planning a tile back splash.

Over time, houses become sentient beings and are able to heal themselves. Just like a cut on your hand will scab over and heal, cracks and gaps in houses will seal themselves. That's why renovations are viewed by many as being unnecessarily cruel. Can you imagine having your insides ripped out? Either that or it's what duluthinbloom said.

The gap at the edge is not the real problem. That’s worrying about the color of the band aid over a nicked artery. That is not a job from a professional qualified tile contractor. One issue in getting recommendations from homeowners is that most homeowners don’t actually know enough to be able to judge if the job was done correctly. All they know is the guy mostly showed up on time and it isn’t too obviously a fail. They do not understand that what lies beneath their pretty and expensive tile is far more expensive, and far more important, than what it ends up looking like in it’s superficial appearance. Not meaning to insult you, but that’s true here. You are noticing the superficial low quality installation aesthetic appearance, but do not have any specifics about any of the processes or materials underneath the spot bonded tile on the out of plane walls, with no visible waterproofing on the still visible wall underneath. You’re still worried about the superficial appearance, and that’s not the real issue here. If that visible gap didn’t exit, you’d never think about the other deficiencies that are pretty apparent to an experienced eye because of that gap. (You’re actually very lucky the gap is there!) And you might go on to recommend this hack to other homeowners. Only to retract that recommendation in a couple of years time when the mold shows up. After he’s done his damage to other projects, and put you in his rear view mirror. Homeowner recommendations for skilled labor projects should always be taken with a huge dose of skepticism. Professional recommendations, only slightly more weight. An electrician doesn’t understand tile. The weight to give are industry certifying bodies, and membership in trade specific groups. Industry education for tile is trying to catch up and police itself of the unqualified. Its not the $2 a square foot Depot lot guys that are to be worried about. People know they’re getting nothing quality for that. It’s the 2K tub wall guys that are the problem. They present as knowledgeable, but aren’t. They do far more damage than the day labor guys, because people have higher expectations, but don’t have the personal knowledge to verify that the contractor can fulfill those expectations. A homeowner shouldn’t have to learn all about the technical details. But in the Wild Wild West of guys with trowels for hire, that learned knowledge is the bottom line defense of their castles against Trojan Horse contractor’s who surface appear to be qualified, but aren’t. To mix a half dozen metaphors. That work is an obvious fail. It needs to be completely redone, from the framing out, with someone who understands and follows industry standards. While cost is no guarantee of quality work, it is an overall indication of the time and quality of materials being used on the job, which does correlate to the quality of the work. I‘m not talking the actual tile. That’s completely irrelevant to what goes on under the tile. The required materials and time that it takes to do a shower properly has the industry prevailing costs be in the 7-9K range. A tub surround, 4-7K. When interviewing for your new contractor, after taking the proper steps to fire this one, please use the questions from the CTEF website about how to find a quality tile installer. Then use your now educated sad experience to educate others.

Yes there should something to cover that area because if your tiles do not cover the the window will not look good and the finishing will not be neat and proper.