Migrating a Chrome extension from MV2 to MV3: what broke, how I fixed it, and what I'd do differently

Google's Manifest V3 deadline is here. If you have a Chrome extension still running on MV2, you're on borrowed time. I migrated PR Focus (and two other extensions) from MV2 to MV3. Nothing broke catastrophically — but several things surprised me. Here's what I learned, so you don't have to figure it