ENH: IntersectionObserver at 1/3 screen (#567)

* IntersectionObserver at 1/3 screen

* Update src/sphinx_book_theme/assets/scripts/index.js

* Filesystem

Author: choldgraf

src/sphinx_book_theme/assets/scripts/index.js

@@ -131,7 +131,11 @@ var initTocHide = () => {
   };
 
   // Set up the intersection observer to watch all margin content
-  let tocObserver = new IntersectionObserver(hideTocCallback);
+  let options = {
+    // Trigger callback when the top of a margin item is 1/3 up the screen
+    rootMargin: "0px 0px -33% 0px",
+  };
+  let tocObserver = new IntersectionObserver(hideTocCallback, options);
   // TODO: deprecate popout after v0.5.0
   const selectorClasses = [
     "marginnote",