Adjust scrolling further

http/poem.html

@@ -200,7 +200,7 @@
                     audio.play();
                     audio.classList.add("playing");
                     button.classList.add("playing");
-                    startAutoScroll(emToPx(0.5) * 20 / 12);
+                    startAutoScroll(0.5);
                 }
                 else {
                     audio.pause();
@@ -213,16 +213,36 @@
 
             let scrollAnimationHandle = null;
 
-            function startAutoScroll(speed) {
+            function scrollMaxValue() {
+              const body = document.body;
+              const html = document.documentElement;
+              const documentHeight = Math.max(
+                body.scrollHeight,
+                body.offsetHeight,
+                html.clientHeight,
+                html.scrollHeight,
+                html.offsetHeight
+              );
+              const windowHeight = window.innerHeight;
+              return documentHeight - windowHeight;
+            };
+
+            function startAutoScroll(emsPerTick) {
                 stopAutoScroll();
-                const startY = window.scrollY;
-                const startTime = performance.now();
-                function step() {
-                    const currentTime = performance.now();
-                    const elapsed = (currentTime - startTime) / 1000;
-                    const newY = Math.min(startY + (elapsed * speed), window.scrollMaxY);
-                    window.scrollTo({ top: newY });
-                    if (newY < window.scrollMaxY) {
+                let lastTime = null;
+                let scrollFraction = 0;
+                function step(currentTime) {
+                    const deltaTime = lastTime == null ? 0 : currentTime - lastTime;
+                    lastTime = currentTime;
+                    const elapsedSeconds = deltaTime / 1000;
+                    const pixelsPerSecond = emToPx(emsPerTick) / 12 * 20;
+                    const maxY = scrollMaxValue();
+                    let newY = Math.min(window.scrollY + (elapsedSeconds * pixelsPerSecond), maxY);
+                    scrollFraction += newY % 1;
+                    newY += Math.floor(scrollFraction);
+                    scrollFraction %= 1;
+                    window.scrollTo({ top: Math.floor(newY) });
+                    if (newY < maxY) {
                         scrollAnimationHandle = requestAnimationFrame(step);
                     }
                 }
@@ -234,9 +254,6 @@
                   cancelAnimationFrame(scrollAnimationHandle);
               }
             }
-
-            window.addEventListener("wheel", stopAutoScroll, { passive: true });
-            window.addEventListener("touchmove", stopAutoScroll, { passive: true });
         </script>
         <script type="x-shader/x-fragment" id="end-portal-vsh">
             attribute vec2 a_position;