I am not the author of this snippet, but I found it useful (tested on Chrome and Safari):

javascript:(function(){
  allowCopyAndPaste = function(e){
  e.stopImmediatePropagation();
  return true;
  };
  document.addEventListener('copy', allowCopyAndPaste, true);
  document.addEventListener('paste', allowCopyAndPaste, true);
  document.addEventListener('onpaste', allowCopyAndPaste, true);
})(); 

Original: https://gist.github.com/Gustavo-Kuze/32959786ce55b2c3751629e40c75c935