diff --git a/style/main.css b/style/main.css
index 46d6f49..7fc537c 100644
--- a/style/main.css
+++ b/style/main.css
@@ -153,16 +153,16 @@ main.loading, main.not-found {
   position: relative;
   max-width: none;
   width: 100%;
-  /* svh (small viewport height): a fixed, stable length - computed
-     once, never recalculated mid-scroll as mobile Safari's address
-     bar hides/shows, unlike plain vh/dvh on some engines. Pinned to
-     the *smallest* possible viewport (toolbar visible) so the box
-     never needs to grow into space the toolbar might reclaim.
-     yes.js's canvas measures this box's own rendered rect (not
-     window.innerWidth/innerHeight) via ResizeObserver, so the canvas
-     always matches whatever height this resolves to - the two can't
-     disagree, which is what was actually causing the content below to
-     visibly jump.
+  /* svh here is only the pre-JS/no-JS fallback (and first paint before
+     yes.js's constructor runs). Once yes.js mounts, it overwrites this
+     with an inline `height: <px>` frozen from a single measurement -
+     see setupCanvas()'s comment in yes.js for why: on real mobile
+     Safari, content bottom-aligned inside this box kept sliding down
+     as the address bar collapsed even with a spec'd-stable viewport
+     unit here, so the box's actual height can't be trusted to stay
+     put on that unit alone. An inline style set from JS always wins
+     the cascade over this rule, so that frozen number is what
+     actually governs once the page is interactive.
      Plain-vh fallback declared first - an engine without svh support
      ignores the invalid second line rather than falling through to
      auto height, which would collapse this to the height of its
diff --git a/yes.js b/yes.js
index 9c9ce3c..5061c84 100644
--- a/yes.js
+++ b/yes.js
@@ -41,8 +41,13 @@ export class RasterizedYES {
         this.containmentStrength = 0.5;
         this.wiggleAmount = 0.5;
 
+        // .hero-canvas is inset:0 inside this - freezing an inline
+        // height here (below) is what actually locks the box, not
+        // just reading its rect.
+        this.heroEl = this.rasterCanvas.closest('.hero');
+
         this.setupCanvas();
-        this.setupResizeObserver();
+        this.setupResizeHandler();
         this.setupMouseTracking();
         this.setupClickHandler();
         this.rasterizeText();
@@ -52,24 +57,34 @@ export class RasterizedYES {
 
     stop() {
         this.destroyed = true;
-        if (this._resizeObserver) {
-            this._resizeObserver.disconnect();
+        if (this._resizeHandler) {
+            window.removeEventListener('resize', this._resizeHandler);
         }
     }
 
-    // Measures the canvases' own containing box (.hero-canvas, which
-    // tracks .hero-yes via inset:0) rather than window.innerWidth/
-    // innerHeight. window.innerHeight is the live viewport - it moves
-    // as mobile Safari's address bar hides/shows on scroll, which
-    // doesn't itself match whatever fixed value .hero-yes's CSS height
-    // is pinned to. Sizing the canvas off the box's own rect instead
-    // means the two can never disagree, whichever viewport unit ends
-    // up governing that box.
+    // Reading .hero-canvas's rendered rect (a prior attempt at this)
+    // only helps if that rect actually stays put - and on real mobile
+    // Safari it didn't: .hero-copy (bottom-aligned via flexbox inside
+    // .hero-yes) kept sliding down as the address bar collapsed, even
+    // with a spec'd-stable viewport unit (svh, then lvh) driving the
+    // box's height. Either the browser isn't holding up its end, or
+    // something in the cascade is still landing on a live value - not
+    // provable from here without the device. Rather than keep
+    // guessing at which CSS viewport unit actually holds still, yes.js
+    // becomes the source of truth instead: it freezes .hero-yes's
+    // rendered height to a literal inline px value once, up front. An
+    // inline style always wins the cascade over the stylesheet's
+    // `height: 100svh`, so once this runs, nothing the browser does
+    // with that unit afterward can move the box - the number is fixed
+    // in the DOM, not recomputed from a unit at all.
     setupCanvas() {
         const pixelRatio = window.devicePixelRatio || 1;
-        const rect = this.rasterCanvas.parentElement.getBoundingClientRect();
-        const width = rect.width;
-        const height = rect.height;
+        const width = window.innerWidth;
+        const height = window.innerHeight;
+
+        if (this.heroEl) {
+            this.heroEl.style.height = height + 'px';
+        }
 
         this.rasterCanvas.style.width = width + 'px';
         this.rasterCanvas.style.height = height + 'px';
@@ -89,19 +104,19 @@ export class RasterizedYES {
         this.pixelRatio = pixelRatio;
     }
 
-    // ResizeObserver reacts to the container's box actually changing
-    // size - a fixed-height box (svh, lvh, or plain px) never fires
-    // this at all during the address-bar animation, so there's no
-    // need for the old "only redraw if width changed" heuristic that
-    // guessed at which resize events were real.
-    setupResizeObserver() {
-        this._resizeObserver = new ResizeObserver((entries) => {
-            const { width, height } = entries[0].contentRect;
-            if (width === this.displayWidth && height === this.displayHeight) return;
+    setupResizeHandler() {
+        // Gate on width, not height: mobile Safari's address-bar
+        // animation changes window.innerHeight continuously with no
+        // real layout change to react to (that's the live value this
+        // whole method exists to stop trusting). A genuine resize -
+        // orientation change, desktop window drag - always changes
+        // width too, so that's the real signal to re-freeze on.
+        this._resizeHandler = () => {
+            if (window.innerWidth === this.displayWidth) return;
             this.setupCanvas();
             this.rasterizeText();
-        });
-        this._resizeObserver.observe(this.rasterCanvas.parentElement);
+        };
+        window.addEventListener('resize', this._resizeHandler);
     }
 
     setupMouseTracking() {