For those pesky websites that disable scrolling. Fiend mode button available in the popup if you dare. Install using the following link: https://gist.githubusercontent.com/Erquint/051e173afc07440050ac04c493d4c8d1/raw/scrollfiend.user.js

scrollfiend.user.js

// ==UserScript==
// @name Scrollfiend
// @description Enables scrolling on those pesky websites that disable it. Engage fiend mode with a popup button if you dare.
// @version 1.1.0
// @namespace gness.na@gmail.com
// @author https://gist.github.com/Erquint
// @homepageURL https://gist.github.com/Erquint/051e173afc07440050ac04c493d4c8d1
// @updateURL https://gist.githubusercontent.com/Erquint/051e173afc07440050ac04c493d4c8d1/raw/scrollfiend.user.js
// @downloadURL https://gist.githubusercontent.com/Erquint/051e173afc07440050ac04c493d4c8d1/raw/scrollfiend.user.js
// @match *://*/*
// @grant GM_addStyle
// @grant GM_registerMenuCommand
// @run-at document-idle
// ==/UserScript==

// StyleSheetList[CSSStyleSheet] sheets = document.styleSheets
// CSSRuleList[CSSRule]          rules  = CSSStyleSheet.cssRules
// CSSRuleList[CSSRule]          rules  = CSSRule.cssRules
// CSSStyleDeclaration           style  = CSSRule.style

function get_all_rules(){
  buffer_1 = Array.from(document.styleSheets);

  while(true){
    let buffer_2 = new Array;
    let sheets_found = false;
    while(buffer_1.length){
      sheet_or_rule = buffer_1.pop();
      if(!!sheet_or_rule.cssRules && !!sheet_or_rule.cssRules.length){ // Sheet.
        sheets_found = true;
        buffer_2.push(...sheet_or_rule.cssRules);
      }else{ // Rule.
        buffer_2.push(sheet_or_rule);
      }
    }

    if(sheets_found){
      buffer_1 = buffer_2;
    }else{ // All page rules are in buffer_2.
      return buffer_2;
    }
  }
}

function undisable_scrollbars(){
  for(rule of rules){
    if(!!rule.style){
      rule.style.overflowX = 'unset';
      rule.style.overflowY = 'unset';
    }
  }
}

function fiend_mode(){
  for(rule of rules){
    if(!!rule.style){
      rule.style.overflowX = 'scroll';
      rule.style.overflowY = 'scroll';
      style = 'html,body,main,*{overflow:unset !important}'
    }
  }
}

let rules = get_all_rules();
undisable_scrollbars();
rules.push(GM_addStyle('html,body,main,*{overflow:unset !important}').sheet.cssRules[0]);
GM_registerMenuCommand('Engage fiend mode!', fiend_mode)