To fix Safari flickering force GPU acceleration by adding transform translate3d(0,0,0) to your element (via https://muffinman.io/ios-safari-scroll-position-fixed/)

safari-flicker-fix.css

/*
If you ever had to fix element on scroll,
you probably had an issue on iOS Safari
(and other mobile devices). Element will
usually flicker, and disappear until scrolling
has stopped completely.

Just force GPU acceleration by adding transform:
translate3d(0,0,0); to your element.
*/

.Element-header {
  transform: translate3d(0,0,0);
}

.Element-header--fixed {
  top: 0;
  position: fixed;
}