fix: fix GM_setClipboard on Firefox

close #251

src/injected/content/clipboard.js

@@ -1,15 +1,7 @@
-let textarea;
 let clipboardData;
 
-function init() {
-  textarea = document.createElement('textarea');
-  textarea.style.position = 'absolute';
-  textarea.style.width = 0;
-  textarea.style.height = 0;
-  textarea.style.left = '-20px';
-}
-
 function onCopy(e) {
+  e.stopImmediatePropagation();
   e.preventDefault();
   const { type, data } = clipboardData;
   e.clipboardData.setData(type || 'text/plain', data);
@@ -17,12 +9,8 @@ function onCopy(e) {
 
 export default function setClipboard({ type, data }) {
   clipboardData = { type, data };
-  if (!textarea) init();
   document.addEventListener('copy', onCopy, false);
-  document.documentElement.appendChild(textarea);
-  textarea.focus();
   const ret = document.execCommand('copy', false, null);
-  document.documentElement.removeChild(textarea);
   document.removeEventListener('copy', onCopy, false);
   if (!ret && process.env.DEBUG) {
     console.warn('Copy failed!');