Fixing the scroll function and adding docs

src/utils/scroll-to.js

@@ -29,21 +29,37 @@ function getScrollableContainer() {
   return scrollableContainer;
 }
 
+/**
+ * Smoothly scrolls to a given Y position using Easing.Swing. It'll run a
+ * callback upon finishing.
+ * @param {number} y Offset of the page to scroll to.
+ * @param {number} duration Duration of the animation. 500ms by default.
+ * @param {function} cb Callback function to call upon completion.
+ */
 function scrollTo(y, duration = 500, cb = () => {}) {
   const scrollableContainer = getScrollableContainer();
   const delta = y - scrollableContainer.scrollTop;
-  const increment = 20;
+  const startLocation = scrollableContainer.scrollTop;
+  const increment = 16;
+
+  if (!duration) {
+    scrollableContainer.scrollTop = y;
+    cb();
+    return;
+  }
 
   const animateScroll = elapsedTime => {
     elapsedTime += increment;
-    const percent = elapsedTime / duration;
+    const percent = Math.min(1, elapsedTime / duration);
+    const easingP = Easings.swing(
+      percent,
+      elapsedTime * percent,
+      y,
+      delta,
+      duration);
 
-    scrollableContainer.scrollTop = Easings.swing(
+    scrollableContainer.scrollTop = Math.floor(startLocation +
-        percent,
+        (easingP * delta));
-        elapsedTime * percent,
-        y,
-        delta,
-        duration) * delta;
 
     if (elapsedTime < duration) {
       setTimeout(() => animateScroll(elapsedTime), increment);