nav: animate Projects/Blog underlines + coherent directional slide

- Projects/Blog had no data-nav-id so the scroll-spy skipped them entirely;
  point them at the homepage #projects/#writing sections so all five animate
- Drive the underline declaratively (re-assert all on each change) so none can
  be left stranded mid-spring
- Slide follows scroll direction: activating underline grows from its trailing
  edge while the rest collapse toward the leading edge (down→right, up→left)

src/components/Nav.astro

@@ -2,11 +2,15 @@
 import ThemeToggle from "./ThemeToggle.astro";
 import { site } from "../data/site";
 
+// `id` ties a nav link to a homepage section so the Motion layer animates its
+// underline as that section scrolls through (scroll-spy). Projects/Blog navigate
+// to their own pages on click, but on the homepage they track the #projects and
+// #writing sections so every nav item gets the underline — in both directions.
 const links = [
   { label: "About", href: "/#about", id: "about" },
   { label: "Skills", href: "/#skills", id: "skills" },
-  { label: "Projects", href: "/projects/", id: null },
-  { label: "Blog", href: "/blog/", id: null },
+  { label: "Projects", href: "/projects/", id: "projects" },
+  { label: "Blog", href: "/blog/", id: "writing" },
   { label: "Contact", href: "/#contact", id: "contact" },
 ];
 ---

src/scripts/anim.ts

@@ -97,45 +97,61 @@ function initNav(): void {
   if (!links.length) return;
 
   const spring = { type: "spring", stiffness: 380, damping: 30 } as const;
-  let active: HTMLAnchorElement | null = null;
+  let activeIdx = -1;
 
-  const under = (a: HTMLAnchorElement, on: boolean) => {
+  // `fromRight` flips the grow/collapse origin so the underline animates in the
+  // direction of travel: scrolling down it grows from the left (moves right);
+  // scrolling back up it grows from the right (moves left).
+  const under = (a: HTMLAnchorElement, on: boolean, fromRight = false): void => {
     const ul = a.querySelector<HTMLElement>(".nav__ul");
-    if (ul) animate(ul, { scaleX: on ? 1 : 0 }, spring);
+    if (!ul) return;
+    ul.style.transformOrigin = fromRight ? "100% 50%" : "0% 50%";
+    animate(ul, { scaleX: on ? 1 : 0 }, spring);
   };
 
-  for (const a of links) {
-    a.addEventListener("pointerenter", () => under(a, true));
-    a.addEventListener("pointerleave", () => under(a, a === active));
-  }
-
-  // Light up the link whose section is currently being read. Driven straight off
-  // scroll position (recomputed every frame) so it tracks BOTH directions — the
-  // old per-section inView(enter) fired only on the way DOWN and went dead on the
-  // way back up. Homepage anchor links only (Projects/Blog have no section here).
+  // Links that map to a section ON THIS PAGE. Projects/Blog point at the homepage
+  // #projects / #writing sections so every nav item gets the underline; on their
+  // own pages those sections don't exist and they simply drop out of the spy.
   const sectioned = links.filter(
     (a) => a.dataset.navId && document.getElementById(a.dataset.navId),
   );
+
+  for (const a of links) {
+    a.addEventListener("pointerenter", () => under(a, true));
+    a.addEventListener("pointerleave", () =>
+      under(a, sectioned.indexOf(a) === activeIdx),
+    );
+  }
+
+  // Light up the link whose section is currently being read, recomputed from
+  // scroll position so it tracks BOTH directions. Every change re-asserts ALL
+  // underlines (active → 1, the rest → 0) so none can be left stranded, and the
+  // grow/collapse origin follows the scroll direction.
   if (sectioned.length) {
     const setActive = (): void => {
       const vh = window.innerHeight;
-      const ref = vh * 0.45; // the section whose top has passed this reading line is "active"
-      let current: HTMLAnchorElement | null = null;
-      for (const a of sectioned) {
+      const ref = vh * 0.45; // a section whose top has passed this line is "active"
+      let idx = -1;
+      sectioned.forEach((a, i) => {
         const sec = document.getElementById(a.dataset.navId as string);
-        if (sec && sec.getBoundingClientRect().top <= ref) current = a;
-      }
+        if (sec && sec.getBoundingClientRect().top <= ref) idx = i;
+      });
       // At the very bottom the last (often short) section may never push its top
-      // above the ref line — force it, so Contact lights up at the end instead of
-      // the highlight getting stuck on the previous section (Skills).
+      // above the line — force it so the final item lights up.
       if (window.scrollY + vh >= document.documentElement.scrollHeight - 4) {
-        current = sectioned[sectioned.length - 1];
-      }
-      if (current !== active) {
-        if (active) under(active, false);
-        active = current;
-        if (current) under(current, true);
+        idx = sectioned.length - 1;
       }
+      if (idx === activeIdx) return;
+      const backward = idx < activeIdx; // scrolling up → travel left
+      activeIdx = idx;
+      // Coherent directional slide: the activating underline grows from its
+      // trailing edge while the others collapse toward the leading edge, so the
+      // whole motion follows the scroll direction (down → right, up → left)
+      // instead of the active/old pair diverging.
+      sectioned.forEach((a, i) => {
+        const on = i === idx;
+        under(a, on, on ? backward : !backward);
+      });
     };
     scroll(setActive);
     setActive();