compose.js

document.addEventListener('DOMNodeInserted', function(e){
  if(e.target.classList.contains('An')){
    addButton(e.target);
  }
});
$('.An').forEach(addButton);

function addButton(composeNode){
  var button = $('<div class="wG J-Z-I"><div class="J-J5-Ji J-Z-I-Kv-H"><div class="J-J5-Ji J-Z-I-J6-H"><div class="aA7 aaA aMZ"><img src="//composeIcon" /></div></div></div></div>');
  button.click(function(e){
    //30 lines of code to insert "Hello World" into the right position
  });

  button.hover(
    function(){button.addClass('J-Z-I-JW');}, 
    function(){button.removeClass('J-Z-I-JW');}
  )

  composeWindow.find('.gU.Up  > .J-J5-Ji').append(button);
}