For those interested, i think i found how to solve the problem of
scrolling. I don't know if it's the best way to proceed but for me it
seems to work fine :
in paintweb.src.js at the lines 4558, after the if...else condition :
if ('layerX' in ev) {
      if (_self.image.canvasScale === 1) {
        _self.mouse.x = ev.layerX;
        _self.mouse.y = ev.layerY;
      } else {
        _self.mouse.x = MathRound(ev.layerX /
_self.image.canvasScale);
        _self.mouse.y = MathRound(ev.layerY /
_self.image.canvasScale);
      }
    } else if ('offsetX' in ev) {
      if (_self.image.canvasScale === 1) {
        _self.mouse.x = ev.offsetX;
        _self.mouse.y = ev.offsetY;
      } else {
        _self.mouse.x = MathRound(ev.offsetX /
_self.image.canvasScale);
        _self.mouse.y = MathRound(ev.offsetY /
_self.image.canvasScale);
      }
    }
Add this condition juste after :
    if (navigator.appName=='Microsoft Internet Explorer')
    	_self.mouse.y -= document.documentElement.scrollTop;
I hope it can help someone...
Otherwise, i still have the issue with the color picker, but it's not
so important. Paintweb is really cool, thanks to the author!