Fix flickering header drop shadow (#884)

On iOS Safari, headerTop can sometimes be negative when scrolling and
the comparison to 0 is no longer true, which removes the drop shadow and
causes the flickering.

Remove the comparison with the header position to avoid DOM layout reads
and fix the flickering by not comparing to exactly 0.

Author: holesch

src/furo/assets/scripts/furo.js

@@ -9,9 +9,7 @@ var lastScrollTop = document.documentElement.scrollTop;
 const GO_TO_TOP_OFFSET = 64;
 
 function scrollHandlerForHeader(positionY) {
-  const headerTop = Math.floor(header.getBoundingClientRect().top);
-
-  if (headerTop == 0 && positionY != headerTop) {
+  if (positionY > 0) {
     header.classList.add("scrolled");
   } else {
     header.classList.remove("scrolled");