Sì, è stabile. Riferimento dal codice sorgente Scala:
https://github.com/scala/scala/blob/2.11.x/src/library/scala/collection/SeqLike.scala#L627
def sortBy[B](f: A => B)(implicit ord: Ordering[B]): Repr = sorted(ord on f)
/** Sorts this $coll according to an Ordering.
*
* The sort is stable. That is, elements that are equal (as determined by
* `lt`) appear in the same order in the sorted sequence as in the original.
*
* @see [[scala.math.Ordering]]
*
* @param ord the ordering to be used to compare elements.
* @return a $coll consisting of the elements of this $coll
* sorted according to the ordering `ord`.
*/
def sorted[B >: A](implicit ord: Ordering[B]): Repr = {
val len = this.length
val b = newBuilder
if (len == 1) b ++= this
else if (len > 1) {
b.sizeHint(len)
val arr = new Array[AnyRef](len) // Previously used ArraySeq for more compact but slower code
var i = 0
for (x <- this) {
arr(i) = x.asInstanceOf[AnyRef]
i += 1
}
java.util.Arrays.sort(arr, ord.asInstanceOf[Ordering[Object]])
i = 0
while (i < arr.length) {
b += arr(i).asInstanceOf[A]
i += 1
}
}
b.result()
}
fonte
2016-02-12 18:15:01