Yes laurent, you can set black color to the layer so you will see black instead of white on borders ... if you want something more beautiful use a TiledBackground with texture and put in at the bottom of the layer.
Yes it's possible to make pan stop on piece drag ... overload the event for scrolling and add just under "is in touch" "not is dragging piece" (=> "is dragging piece" ("invert" mode)) ...
I update the sample project with all this settings to show you
always the same link => Here
* Add vertical scrolling and scrolling only if "piece" is not dragging
* Add Black color to layer
* Add TiledBackground with Wave Texture + Sine Opacity wave in/out effect
* Reduce opacity of the turtle sprite to see Sine Opacity wave in/out effect under it.
You can do great things with a few lines as you can see ... you just need more practice !