@olafurpg Exactly the thing you seem to want to work on, full support for mixed Scala/Java projects (or even pure Java projects, built with 'sbt')
And of course, the thing that has been working in Scala IDE + sbteclipse all this time, ProjectRef (source based) dependencies.
Having those 2 would probably be part of making it viable to move existing projects to VSCode + Metals, permitting that 2.13 step towards 3.
When on Scala IDE so far and using either, it looks like you stuck ATM.
scala.App. Finally, when I try to run it, I get the same error listed here.