Upgrading a Legacy Monorepo to React 18: What Nobody Warns You About

React 18 shipped in March 2022. We merged our upgrade in April 2026. If you do that math, yes — we were four years behind. This is a post about what that upgrade actually looked like across a large, multi-generational codebase, and the specific issues that ate most of the time. Spoiler: the "breaking changes" in the official React 18 migration guide are the easy part. The Starting Point Our…