This opts for checking scalafmt when a PR is opened rather than doing it on compile. This can be considered a subjective change however here is my personal reasoning

• Doing format on compile significantly impacts IDE/editor performance when working on the codebase. Since editors/IDE's index/track source files, its more ideal to do this once then every time you compile (especially for a language like Scala where you do frequently compile just to check your code is correct). Furthemore some editors such as Intellij can sometimes report file conflicts when a formatted file is quickly edited.
• Doing a double check of both formatting and checking on compile can be considered redundant. In terms of practicality you don't really care that much if a PR in progress is not formatted, you only want to make sure that code is formatted in the final step before merging.

Note that the PR removes using scalafmt in ci.yaml and instead creates it as a separate pipeline that uses scalafmt-native. This has the following advantages

• scalafmt-native is significantly faster than standard scalafmt in sbt (I haven't seen it ever take longer than 10 seconds on massive codebases)
• it runs in parallel to the main compile/test rather than sequentially. For the same reason it also doesn't block the build/test if your code is formatted (it only blocks the merging of the PR). This gives you quick feedback that the code isn't formatted.

Note that I have submitted this kind of scalafmt checking on numerous Scala projects and there haven't been any issues.