Avoid using Element.classList in DOM surveyor

Related issue:
- uBlockOrigin/uBlock-issues#2076

src/js/contentscript.js

@@ -1024,7 +1024,6 @@ vAPI.DOMFilterer = class {
  // Extract all classes/ids: these will be passed to the cosmetic
  // filtering engine, and in return we will obtain only the relevant
  // CSS selectors.
- const reWhitespace = /\s/;
 
  // https://github.com/gorhill/uBlock/issues/672
  // http://www.w3.org/TR/2014/REC-html5-20141028/infrastructure.html#space-separated-tokens
@@ -1039,19 +1038,21 @@ vAPI.DOMFilterer = class {
  queriedIds.add(s);
  };
 
+ // https://github.com/uBlockOrigin/uBlock-issues/discussions/2076
+ // Performance: avoid using Element.classList
  const classesFromNode = (node, out) => {
- const s = node.className;
- if ( typeof s !== 'string' || s.length === 0 ) { return; }
- if ( reWhitespace.test(s) === false ) {
-  if ( queriedClasses.has(s) ) { return; }
- out.push(s);
- queriedClasses.add(s);
- return;
- }
- for ( const s of node.classList.values() ) {
- if ( queriedClasses.has(s) ) { continue; }
- out.push(s);
- queriedClasses.add(s);
+ const s = node.getAttribute('class');
+ if ( typeof s !== 'string' ) { return; }
+ const len = s.length;
+ for ( let beg = 0, end = 0, token = ''; beg < len; beg += 1 ) {
+ end = s.indexOf(' ', beg);
+ if ( end === beg ) { continue; }
+ if ( end === -1 ) { end = len; }
+  token = s.slice(beg, end);
+  beg = end;
+ if ( queriedClasses.has(token) ) { continue; }
+ out.push(token);
+ queriedClasses.add(token);
  }
  };