protected void whenClicked() {
    if (! showScroll) return ;
    final float mX = myHUD.mouseX(), mY = myHUD.mouseY() ;
    if (grabArea.contains(mX, mY)) {
      initScrollPos = scrollPos ;
    }
    else {
      initScrollPos = -1 ;
      final float inc = Math.max(
        MIN_SCROLL_DIST / (mapArea.ydim() - ydim()), 0.1f
      ) ;
      if (mY > grabArea.ymax()) scrollPos += inc ;
      if (mY < grabArea.ypos()) scrollPos -= inc ;
      if (scrollPos < 0) scrollPos = 0 ;
      if (scrollPos > 1) scrollPos = 1 ;
    }
  }