header {
  background: lightgreen;

  /* these trigger a stacking context */
  position: relative;
  z-index: 1;

  /* but also could be ANY of these, for example...
  
  opacity: 0.99;
  mix-blend-mode: darken; 
  contain: layout;
  isolation: isolate;
*/
}

.big-thing {
  position: absolute;
  right: 30px;
  top: 10px;
  width: 110px;
  height: 110px;
  background: white;
  border-radius: 50%;
  display: grid;
  place-items: center;
  z-index: 9999;
}

main {
  position: relative;
  z-index: 2;
  background: tan;
}

body {
  margin: 0;
  block-size: 100vh;
  display: grid;
  grid-template-rows: 80px 1fr;
}