diff --git a/src/components/CornerNav.astro b/src/components/CornerNav.astro
index cf24139..724b3d9 100644
--- a/src/components/CornerNav.astro
+++ b/src/components/CornerNav.astro
@@ -427,11 +427,15 @@ const items: Item[] = [
     const mode = nav.dataset.navMode === 'light' ? 'light' : 'scroll';
 
     let onScroll: (() => void) | null = null;
+    // Hoisted to the teardown's scope, not the branch's: the re-measure listener and the content observer must
+    // be removable, or every View Transition would stack another observer on the same <main>.
+    let onResize: (() => void) | null = null;
+    let zoneRO: ResizeObserver | null = null;
     if (mode === 'light' || !descent) {
       nav.dataset.zone = 'light';
     } else {
-      // document-y behind the nav's vertical center (fixed near the top edge)
-      const probe = () => (nav.getBoundingClientRect().top + nav.getBoundingClientRect().bottom) / 2;
+      // (The old per-frame `probe()` is gone: it read the nav's rect TWICE on every frame to find a value that
+      // cannot change while scrolling, since the nav is position:fixed. It is now navCenterY, measured once.)
 
       /**
        * Relative luminance of the first OPAQUE thing actually painted behind the nav, or null if the sky is
@@ -447,10 +451,31 @@ const items: Item[] = [
        * and it keeps working when panels move or new ones are added. elementsFromPoint (plural) is used
        * because the nav itself is the topmost thing at that point; its own subtree is skipped.
        */
-      const backdropLuminance = (): number | null => {
+      // ── MEASURED ONCE, NOT PER FRAME. This is the fix for a regression I shipped: the owner reported the
+      // slide transitions had become "almost like sluggish" in prod, and this function was why.
+      //
+      // It used to run inside the scroll handler, i.e. on EVERY animation frame of every scroll — and a deck
+      // slide transition is a ~500ms smooth scroll, so every frame of it. Each call did a fresh
+      // querySelectorAll and then interleaved getBoundingClientRect() with getComputedStyle() per element,
+      // which is the textbook layout-thrash pattern: each style read forces the layout the previous rect read
+      // just invalidated. The handler it replaced did one offsetHeight and two rect reads.
+      //
+      // Nothing it measures actually changes while you scroll. A panel's DOCUMENT-space top and bottom and its
+      // background colour are fixed; only scrollY moves. So the measurement is hoisted into measureBackdrops()
+      // and the per-frame path becomes pure arithmetic over a cached array — no DOM reads at all.
+      type Band = { top: number; bottom: number; lum: number };
+      let bands: Band[] = [];
+      let navCenterY = 0;
+      let descentH = 0;
+      const lin = (c: number) => { const v = c / 255; return v <= 0.03928 ? v / 12.92 : ((v + 0.055) / 1.055) ** 2.4; };
+
+      const measureBackdrops = (): void => {
         const nr = nav.getBoundingClientRect();
-        const y = (nr.top + nr.bottom) / 2;
+        navCenterY = (nr.top + nr.bottom) / 2;   // the nav is position:fixed, so this is scroll-invariant
+        descentH = descent.offsetHeight;
         const vw = window.innerWidth;
+        const sy = window.scrollY;
+        const next: Band[] = [];
         // GEOMETRY, NOT HIT-TESTING. document.elementsFromPoint looked like the obvious tool and is the wrong
         // one: it SKIPS every element with pointer-events: none, which is precisely what all of this site's
         // decorative sky layers are (the fluid-sky canvas, the atmosphere washes, the hero legibility band).
@@ -463,42 +488,58 @@ const items: Item[] = [
         // only things that hide the sky, they are always `main > section > <block>`, and their boxes are
         // ordinary geometry that no pointer-events value can hide. Anything else means the sky is what shows,
         // and the descent-fraction proxy below is the right model for a sky whose colour varies with depth.
+        // Read every candidate's geometry and colour ONCE, in document coordinates. The rect reads and the
+        // style reads still interleave here, but this runs on init and on resize rather than 60 times a second.
         for (const el of document.querySelectorAll<HTMLElement>('main > section > *')) {
           const r = el.getBoundingClientRect();
-          if (r.top > y || r.bottom < y) continue;          // not on the nav's line
           if (r.width < vw * 0.6) continue;                 // not full-bleed: a card, not a panel
           const parts = /rgba?\(([^)]+)\)/.exec(getComputedStyle(el).backgroundColor);
           if (!parts) continue;
           const [cr, cg, cb, ca = 1] = parts[1].split(',').map((s) => parseFloat(s));
           if (!(ca >= 0.9)) continue;                       // translucent: the sky still reads through
-          const lin = (c: number) => { const v = c / 255; return v <= 0.03928 ? v / 12.92 : ((v + 0.055) / 1.055) ** 2.4; };
-          return 0.2126 * lin(cr) + 0.7152 * lin(cg) + 0.0722 * lin(cb);
+          next.push({
+            top: r.top + sy,
+            bottom: r.bottom + sy,
+            lum: 0.2126 * lin(cr) + 0.7152 * lin(cg) + 0.0722 * lin(cb),
+          });
         }
-        return null;
+        bands = next;
       };
 
       const update = () => {
-        // An opaque backdrop answers the question directly; the gradient fraction is only the fallback for
-        // when the sky itself is what shows through. 0.35 sits between the paper tones (~0.85) and the dusk/
-        // ink ones (~0.02-0.2), so mid-grey dusk still chooses paper text as it did before.
-        const lumBehind = backdropLuminance();
-        if (lumBehind !== null) {
-          nav.dataset.zone = lumBehind > 0.35 ? 'light' : 'dark';
-          return;
+        // THE WHOLE PER-FRAME PATH, and it touches the DOM exactly once (scrollY, which is free — it does not
+        // force layout). An opaque backdrop answers the question directly; the gradient fraction is the
+        // fallback for when the sky itself is what shows through. 0.35 sits between the paper tones (~0.85)
+        // and the dusk/ink ones (~0.02-0.2), so mid-grey dusk still chooses paper text as it did before.
+        const docY = window.scrollY + navCenterY;
+        for (const b of bands) {
+          if (docY >= b.top && docY <= b.bottom) {
+            nav.dataset.zone = b.lum > 0.35 ? 'light' : 'dark';
+            return;
+          }
         }
-        const h = descent.offsetHeight;
-        if (!h) return;
-        const frac = (window.scrollY + probe()) / h;
-        nav.dataset.zone = frac > DARK_FROM ? 'dark' : 'light';
+        if (!descentH) return;
+        nav.dataset.zone = docY / descentH > DARK_FROM ? 'dark' : 'light';
       };
+
+      measureBackdrops();
       let ticking = false;
       onScroll = () => {
         if (ticking) return;
         ticking = true;
         requestAnimationFrame(() => { update(); ticking = false; });
       };
+      // RESIZE RE-MEASURES; SCROLL DOES NOT. Previously both events ran the same handler, which is how the
+      // measurement ended up in the scroll path in the first place.
+      onResize = () => { measureBackdrops(); onScroll?.(); };
       window.addEventListener('scroll', onScroll, { passive: true });
-      window.addEventListener('resize', onScroll, { passive: true });
+      window.addEventListener('resize', onResize, { passive: true });
+      // The document also changes height WITHOUT a resize — the terrain canvas and images settle after first
+      // paint, and a deck slide can grow. Stale bands would leave the nav in the wrong zone, so watch the
+      // content box; this fires rarely and never during a scroll.
+      zoneRO = new ResizeObserver(() => measureBackdrops());
+      const mainEl = document.querySelector('main');
+      if (mainEl) zoneRO.observe(mainEl);
       update();
     }
 
@@ -601,10 +642,9 @@ const items: Item[] = [
     document.addEventListener('keydown', onDocKey);
 
     teardown = () => {
-      if (onScroll) {
-        window.removeEventListener('scroll', onScroll);
-        window.removeEventListener('resize', onScroll);
-      }
+      if (onScroll) window.removeEventListener('scroll', onScroll);
+      if (onResize) window.removeEventListener('resize', onResize);
+      zoneRO?.disconnect();
       toggle?.removeEventListener('click', onToggle);
       menu?.removeEventListener('click', onMenu);
       document.removeEventListener('pointerdown', onDocPointer);