This works perfectly fine! However if I change it to this:

var cancel = false
var i = 0
this.forEach {
    val result = !it(using, interaction)
    println("${i++} cancelled = $result")
    if (result) {
        println("cancelling")
        return
    }
}

It prints out 4 times DOES NOT print cancelling... and then it just stops.

sounds like output buffering, not a compiler issue

I thought so as well, but I've tried manually flushing output stream. Also, I was writing a full stack trace at one point which doesn't have a shot of being fully buffered.