I need this button fixed at the bottom of the parent div and floating over its scrolling content.
I'm really surprised this doesn't work. I expected position: relative on the parent in combination with bottom: 0 on the fixed element to achieve this..
PS: the button must be inside the div.
div {
  position: relative;
  width: 200px;
  height: 100px;
  overflow-y: scroll;
  background-color: lightblue;
}
button {
  position: fixed;
  bottom: 0;
}
body {
  height: 200px;
}<div>
  <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Atque, assumenda. Sint optio, praesentium omnis voluptas facilis nam asperiores quod itaque repellat eaque aut molestias reiciendis quibusdam harum rem, cumque nihil!</p>
  <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Voluptatibus, ab! Tempore sunt eligendi, voluptates quaerat autem reprehenderit perferendis id hic voluptate modi nisi in eaque quasi veniam delectus, voluptatibus quo.</p>
  <button>Bütton</button>
</div> 
    