code: private int performViewPagerScroll(@NonNull MotionEvent event, float pagerDx) {
          int scrollBegin = viewPager.getScrollX();
          lastViewPagerEventX += pagerDx;
          passEventToViewPager(event);
          return scrollBegin - viewPager.getScrollX();
      }
docs: /*
  Manually scrolls ViewPager and returns actual distance at which pager was scrolled.