Reference implementation of methods of class javax.swing.plaf.LayerUI:

 - getScrollableUnitIncrement(JLayer<? extends V>,Rectangle,int,int)
 - getScrollableTracksViewportWidth()
 - getScrollableTracksViewportHeight()
 - getScrollableBlockIncrement(Rectangle, int, int)
 - getPreferredScrollableViewportSize()

delegate invocation to passed JLayer's view if this view is an instance of Scrollable.

For example:

    public int getScrollableUnitIncrement(JLayer<? extends V> l,
                                          Rectangle visibleRect,
                                          int orientation, int direction) {
        if (l.getView() instanceof Scrollable) {
            return ((Scrollable)l.getView()).getScrollableUnitIncrement(
                    visibleRect, orientation, direction);
        }
        return 1;
    }

Seems that this important aspect of behavior might be specified.
The following JCK tests verify mentioned behavior:
api/javax_swing/JLayer/index.html#General[getPreferredScrollableViewportSize_scrollableView]
api/javax_swing/JLayer/index.html#General[getScrollableBlockIncrement_scrollableView]
api/javax_swing/JLayer/index.html#General[getScrollableTracksViewportHeight_scrollableView]
api/javax_swing/JLayer/index.html#General[getScrollableTracksViewportWidth_scrollableView]
api/javax_swing/JLayer/index.html#General[getScrollableUnitIncrement_scrollableView]