The REPL UI code in Dotty comes from ammonite-terminal and lives in https://github.com/lampepfl/dotty/tree/master/compiler/src/dotty/tools/repl/terminal, it works well enough but does not support Windows. This has been recently fixed in Ammonite by using JLine 3 under Windows. We could merge these changes in our version of ammonite-terminal, but if we're going to depend on JLine anyway we could also drop our version of ammonite-terminal and just use JLine directly (unless there's some features of ammonite-terminal we really need).