I solved it with following scala-snippet. Scala's Scrollable is just a fake wrapper, you are supposed to implement the Java-Scrollable...
import javax.swing.JPanel
import javax.swing.{Scrollable => JScrollable}
import javax.swing.BoxLayout
import java.awt.Dimension
import java.awt.Rectangle
import scala.swing.SequentialContainer
import scala.swing.Orientation
import scala.swing.Scrollable
import scala.swing.Panel
class ScrollablePanel extends Panel with SequentialContainer.Wrapper with Scrollable.Wrapper {
  var scrollIncrement: Int = 10
  var blockScrollIncrement: Int = 50
  val allowVerticalScrolling: Boolean   = true
  val allowHorizontalScrolling: Boolean = false
  override lazy val peer = new JPanel with SuperMixin with JScrollable {
    def getPreferredScrollableViewportSize: Dimension =
      getPreferredSize
    def getScrollableTracksViewportHeight: Boolean =
      !allowVerticalScrolling
    def getScrollableTracksViewportWidth: Boolean =
      !allowHorizontalScrolling
    def getScrollableBlockIncrement(visibleRect: Rectangle, orientation: Int, direction: Int): Int =
      scrollIncrement
    def getScrollableUnitIncrement(visibleRect: Rectangle, orientation: Int, direction: Int): Int =
      blockScrollIncrement
  }
  final protected def scrollablePeer: JScrollable = peer
}