withContext calls ThreadContextElement#updateThreadContext with wrong Job in context

[dovchinnikov]

import kotlin.coroutines.CoroutineContext
import kotlinx.coroutines.job
import kotlinx.coroutines.runBlocking
import kotlinx.coroutines.withContext
import kotlinx.coroutines.ThreadContextElement

class MyElement : ThreadContextElement<Unit> {

  companion object Key : CoroutineContext.Key<MyElement>

  override val key: CoroutineContext.Key<*> get() = Key

  override fun updateThreadContext(context: CoroutineContext) {
    println("update ${context.job}") // both times BlockingCoroutine{Active}@18ef96 
  }

  override fun restoreThreadContext(context: CoroutineContext, oldState: Unit) {}
}

fun main() {
  runBlocking(MyElement()) {
    println("outer ${coroutineContext.job}") // BlockingCoroutine{Active}@18ef96
    withContext(MyElement()) {
        println("inner ${coroutineContext.job}") // UndispatchedCoroutine{Active}@2aaf7cc2
    }
  }
}

This happens because withContext calls withCoroutineContext with newContext which doesn't include newly created UndispatchedCoroutine.