body{
  height: 3000px;
  background-image: url(./bg.gif);
}

button{
  position: fixed;
}

div.modal{
  position: fixed;
  display: none;
  width: 100%;
  height: 100%;
  top: 0;
  left: 0;
  background-color: rgba(0,0,0,0.4);
}

div.inner{
  position: relative;
  top:   50px;
  left:  50px;
  width: 300px;
  max-height: 100%;
  overflow-y: auto;
}

div.content{
  height: 2000px;
  background-image: url(./popup.gif);
}