Sticky header that actually sticks

`position: sticky` fails silently when an ancestor has `overflow: hidden`/`auto`, or when no offset is set. Give it a `top` and check the ancestor chain.

.header {
  position: sticky;
  top: 0;
  z-index: 50;
  backdrop-filter: blur(8px);
}

/* breaks sticky children: */
.ancestor { overflow: hidden; }