The problem
Off-the-shelf windows never fit the way they should.
Standard windows from a hardware store almost never match the actual opening in your home. The gaps get filled with excessive sealant, frames sit slightly crooked, and the whole installation looks like a compromise. It might work, but it never looks right. And mirrors that are not measured, cut, and mounted properly are a safety concern and an eyesore. Wonky edges, visible adhesive overflow, and glass that does not sit flush against the wall.