Drag scrolling with right mouse button (disable contextmenu, but only if scrolled)

hand-scroll.user.js

// ==UserScript==
// @name         Hand Scroll
// @namespace    https://gavinhungry.io
// @version      1.5.0
// @description  Drag scrolling with mouse movement
// @author       Gavin Lloyd <gavinhungry@gmail.com>
// @include      *
// @grant        none
// ==/UserScript==

(() => {
  'use strict';

  const SCALE_X = 0.0;
  const SCALE_Y = 1.0;

  const BUTTON = 3; // 1 = left, 2 = middle, 3 = right mouse button
  const CTRL_KEY = false;
  const ALT_KEY = false;
  const SHIFT_KEY = false;
  const META_KEY = false;

  const AUXCLICK = false;

  let el, clientX, clientY;

  let moved;

  let validEvent = e =>
    e.which === BUTTON &&
    e.ctrlKey === CTRL_KEY &&
    e.altKey === ALT_KEY &&
    e.shiftKey === SHIFT_KEY &&
    e.metaKey === META_KEY;

  let getScrollableEl = el => {
    try {
      let overflowY = window.getComputedStyle(el).overflowY;

      return overflowY !== 'visible' && overflowY !== 'hidden' &&
        el !== document.body &&
        el.scrollHeight > el.clientHeight ?
          el : getScrollableEl(el.parentNode);

    } catch(err) {
      return window;
    }
  };

  let mousemove = e => {
    swallow(e);
    moved = true;

    el.scrollBy(SCALE_X * (clientX - e.clientX), SCALE_Y * (clientY - e.clientY));
    clientX = e.clientX;
    clientY = e.clientY;
  };

  let swallow = e => {
    e.preventDefault();
    e.stopPropagation();
  };

  let finish = e => {
    swallow(e);

    document.removeEventListener('mousemove', mousemove, true);
    document.removeEventListener('mouseover', swallow, true);
    document.removeEventListener('mouseenter', swallow, true);
    document.removeEventListener('mouseup', finish, true);
  };

  let preventContextMenu = e => {
      if(moved) swallow(e);
  };

  if (!AUXCLICK) {
    document.addEventListener('auxclick', e => {
      if (!validEvent(e)) {
        return;
      }

      swallow(e);
    });
  }

  document.addEventListener('mousedown', e => {
    if (!validEvent(e)) {
      return;
    }

    swallow(e);
    moved = false;

    el = getScrollableEl(e.target);
    clientX = e.clientX;
    clientY = e.clientY;

    document.addEventListener('mousemove', mousemove, true);
    document.addEventListener('mouseover', swallow, true);
    document.addEventListener('mouseenter', swallow, true);
    document.addEventListener('mouseup', finish, true);
  }, true);

  if (BUTTON === 3) {
    document.addEventListener('contextmenu', preventContextMenu, true);
  }

})();