user script :: autocopy

autocopy.user.js

// ==UserScript==
// @name           autocopy
// @namespace      https://bigfang.github.io
// @description    Auto copy selected text
// @match          *://*/*
// @run-at         document-end
// @grant          GM_setClipboard
// ==/UserScript==

if (typeof GM_setClipboard != 'function') alert('Your UserScript client has no GM_setClipboard support');

document.addEventListener('mouseup',
  (e) => {
    if (e.button != 0 || ['INPUT', 'TEXTAREA'].includes(e.target.tagName)) 
      return;
    let stext = getSelection().toString();
    if (stext) {
      GM_setClipboard(stext)
    }
  }, false);