So I realized my solution has one issue. But I am not sure how to address it. (It may be ok to not worry about it)

• 1. Take for example the following class in the parent project.

class Foo  {
  def foo = "foo"
}

• 2. The following class in the child project

class Bar extends Foo {
  def bar = "bar"
}

• 3. The following tests in the child project

class BarSpec extends FlatSpec with Matchers with OptionValues with Inside with Inspectors {
  it should "return foo" in {
    new Bar().foo shouldEqual "foo"
  }
  it should "return bar" in {
    new Bar().bar shouldEqual "bar"
  }
}

• 4. You run sbt clean coverage test and the parent project now has 0% coverage since it finished quickly and when it finished it did not have any measurements files to look at in the parent project.
• 5. Now add a test file to core like so

class FooSpec extends FlatSpec with Matchers with OptionValues with Inside with Inspectors {
  "this test" should "kill time" in {
    Thread.sleep(10000)
  }
}

• 6. Again you run sbt clean coverage test and the parent project now has 100% coverage since it finished after child project tests and has the measurements files for the parent project from the child project's test run.

So, This results in a non deterministic result in coverage depending on how long the tests will take to run for each project. Also, this behavior where parent project gets some coverage on statements when covered from a child project may not be something that is desirable. And may cause some confusion.

This is a great fix thank you, and to address the non-deterministic behaviour, we might be able to do something in the sbt plugin that runs a report step last.