One of my first scala projects, "http://tryscala.com" had a good run from 2010 - 2017. There are other good options now, https://scalafiddle.io comes to mind, so I'm retiring it - don't visit the address, not sure what you'll find there now!
It was an online scripting/fiddle tool for scala, with content assist, short URL fiddles, forking etc. Here's the screenshot of how it looked before I deactivated it:
You could edit the code and see the result right there, fork it etc.
See also the original Scripster github project