Version: DataTables 3.1.2 (also seen with FixedColumns 6.1.1 / FixedHeader 5.1.1, but they are not the cause). Chrome and Safari on macOS.

Description:
With scrollX: true, a trackpad fling (momentum scroll) on the scroll body no longer glides to the end of the table. It stops dead at a random point, and the user has to scroll again to reach the edge. The same table glides correctly on DataTables 2.3.x.

Cause:
In 2.x, _fnScrollDraw only synced the body from the header when a header cell received focus:

  $('th, td', scrollHead).on('focus', function () {
      scrollBody.scrollLeft = scrollHead.scrollLeft;
  });

In 3.x the header (and footer) have their own scroll.DT listeners that write back to the body:

  scrollHead.on('scroll.DT', () => {
      let scrollLeft = scrollHead.scrollLeft();
      scrollBody.scrollLeft(scrollLeft);
      scrollFoot.scrollLeft(scrollLeft);
  });

Every body scroll sets the header's scrollLeft, the header's scroll event fires on the next frame, and the body's scrollLeft is assigned again. Any programmatic assignment to scrollLeft, even with an unchanged value, cancels the browser's momentum or smooth scroll animation. Where the fling dies depends on frame timing, so the stop position looks random. Guarding the write with if (body.scrollLeft !== head.scrollLeft) does not help, because the header value is already a frame stale by the time its event fires.

Reproduction (no trackpad needed):
On any scrollX table, in the console:

  var body = document.querySelector('div.dt-scroll-body');
  body.scrollLeft = body.scrollWidth;
  setTimeout(function () {
      body.scrollTo({ left: 0, behavior: 'smooth' });
      setTimeout(function () { console.log('ended at', body.scrollLeft); }, 2500);
  }, 400);

On 3.1.2 this logs the starting position (the smooth scroll is cancelled on its first frame). After $(document.querySelector('div.dt-scroll-head')).off('scroll.DT') it logs 0.

Suggested fix:
Restore the 2.x behaviour: don't sync the body from the header's scroll event, only on header cell focus (the header is overflow: hidden, so focus is the only way a user can scroll it).

Workaround we're using:

  $(document).on('init.dt', function (e, settings) {
      var head = settings.scrollHead, body = settings.scrollBody, foot = settings.scrollFoot;
      if (!head || !head.count()) return;
      var headEl = head.get(0), bodyEl = body.get(0);
      $(headEl).off('scroll.DT');
      if (foot && foot.count()) $(foot.get(0)).off('scroll.DT');
      $(headEl).on('focusin', 'th, td', function () { bodyEl.scrollLeft = headEl.scrollLeft; });
  });