dbl_click_edit_slack_message.user.js

javascript:(function(){
  if(window.location["href"].match("slack") == null){ return; }

  var editIfDblClick = function(){
    document.addEventListener('dblclick', function(e){
      var target = e.target;
      if(target.hasAttribute('class', 'message_body')){
        target.closest("ts-message").getElementsByClassName("action_hover_container")[0].querySelector("a[data-action='actions_menu']").click();
        document.getElementById("edit_link").click();
      }
    })
  }

  editIfDblClick();
})();