look: fix scrolling to sections

This fixes:

1. Sections overlapping with navbar when linked to. The fix is dirty,
   it's just a margin.
2. The inability to scroll down to 'contacts' and other lower sections.
   We just expand the second-to-last section to allow the browser to
   scroll.
3. Updating the currently active section - this now uses a global view
   of all visible sections that is updated by the observer diff, thereby
   fixing the logic.

src/components/landing-page.tsx

@@ -51,7 +51,7 @@ function NewSection({
   after?: ReactElement;
 }) {
   return (<section id={id} className="bg-background">
-    <div className="container mx-auto px-4 gap-6 flex flex-col">
+    <div className="container mx-auto px-4 gap-6 flex flex-col mt-20">
       {children}
       {after}
     </div>

src/globals.css

@@ -155,3 +155,8 @@
 .z-max {
   z-index: 10000;
 }
+
+/* Allow to scroll past the last section, needed for fragment-directed navigation. */
+section#contact {
+  min-height: 100vh;
+}

src/hooks/color-sections.tsx

@@ -6,7 +6,7 @@ export const linksOrder: Array<Sections> = [
   "tickets",
   "cfp",
   "details",
-  "contact"
+  "contact",
 ]
 
 export function useColorSections(parent: React.RefObject<HTMLDivElement | null>) {
@@ -27,6 +27,7 @@ export function useColorSections(parent: React.RefObject<HTMLDivElement | null>)
       acc[value] = parent.current!.querySelector('[data-sub="' + value + '"]')!;
       return acc;
     }, {} as Record<Sections, HTMLAnchorElement>);
+    console.log(subs);
     const links = linksOrder.reduce((acc, value) => {
       acc[value] = parent.current!.querySelector('[href="#' + value + '"]')!;
       return acc;
@@ -34,22 +35,32 @@ export function useColorSections(parent: React.RefObject<HTMLDivElement | null>)
 
     console.log(links)
 
-    const observer = new IntersectionObserver((entries) => {
+    // Set of currently intersecting sections by ID.
+    let intersecting: Set<string> = new Set;
 
+    const observer = new IntersectionObserver((entries) => {
+      // Update intersection set based on diff.
+      let startedIntersecting: Set<string> = new Set;
+      let stoppedIntersecting: Set<string> = new Set;
       for (const entry of entries) {
-        const target = entry.target.id as Sections
         if (entry.isIntersecting) {
-          // FIXME: This seems to be VERY broken on firefox.
-          // See: https://bugzilla.mozilla.org/show_bug.cgi?id=1250972
-          // It basically spikes up CPU usage to some enormous values just to update the hash, like WTF firefox.
-          // if (history.replaceState) {
-          //   timeout = setTimeout(() => {
-          //     history.replaceState(null, "", `#${target}`)
-          //   }, 150)
-          // }
+          startedIntersecting.add(entry.target.id);
+        } else {
+          stoppedIntersecting.add(entry.target.id);
+        }
+      }
+      intersecting = intersecting.difference(stoppedIntersecting);
+      intersecting = intersecting.union(startedIntersecting);
+      console.log(intersecting);
+
+      // Act upon intersection set to find the highest intersecting section -
+      // that's our 'active' section.
+      for (const id of linksOrder) {
+        if (intersecting.has(id)) {
+          console.log("best: " + id);
           subs[previous.current]?.classList.remove('scale-x-100');
           links[previous.current]?.classList.remove('text-primary');
-          previous.current = target;
+          previous.current = id;
 
           subs[previous.current]?.classList.add('scale-x-100');
           links[previous.current]?.classList.add('text-primary');