Igor posted great answers ( http://forum.world.st/Constrained-Morph-dragging-td4655743.html )
Your sliding morph works very smoothly. Nice.
Two quick remarks on style.
(1) IMHO don't worry about overriding a private method. There are already 6 overrides of privateFullMoveBy: in Morphic.
(2) It would be nice if your morph responded sensibly to openInWorld. For me, when I tried openInWorld, the morph received privateFullMoveBy: before it had an owner, so isBeingDragged failed.
For archaeology, you could try reverse engineering Slider and ScrollBar. Please contribute code comments, test cases or refactorings back to Pharo or Squeak :)
Have fun! David