public void updatePosition(int dx, int dy) {
    documentScrollX += dx;
    documentScrollY += dy;
    if (!scrolling) scrollThumb();
}

float multiplier2 = 0.0f;
float scrollBarPosition = 0.0f;

void scrollThumb() {
    // the absolute position is only updated when the scrollbar itself
    // gets scrolled by the touch listener
    // so use documentScrollY instead, which is equivilant in that
    // it is maintained by a callback from the document view
    // specifying relative scroll direction
    multiplier2 = documentScrollY / (documentHeight - windowHeight);
    scrollBarPosition = clip.getY();
    scrollBarPosition = multiplier2 * (getHeight() - scrollBarPosition);
    clip.setY(scrollBarPosition);
}