css/css-scroll-snap/scroll-padding.html

<!DOCTYPE html>
<link rel="help" href="https://drafts.csswg.org/css-scroll-snap-1/#scroll-padding" />
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
div {
  position: absolute;
  margin: 0px;
}
#scroller {
  height: 500px;
  width: 500px;
  overflow: hidden;
  scroll-snap-type: both mandatory;
}
#target {
  width: 300px;
  height: 300px;
  background-color: blue;
}
</style>

<div id="scroller">
  <div style="width: 2000px; height: 2000px;"></div>
  <div id="target"></div>
</div>

<script>
test(() => {
  scroller.style.scrollPadding = "100px";

  target.style.scrollSnapAlign = "start";
  target.style.left = "300px";
  target.style.top = "300px";

  scroller.scrollTo(0, 0);
  // `target position (300px, 300px)` - `padding (100px, 100px)`.
  assert_equals(scroller.scrollLeft, 200);
  assert_equals(scroller.scrollTop, 200);

  target.style.scrollSnapAlign = "end";

  // `target position (300px, 300px)` + `target size (300px, 300px) +
  // `padding (100px, 100px) - `scroller size (500px, 500px)`.
  scroller.scrollTo(0, 0);
  assert_equals(scroller.scrollLeft, 200);
  assert_equals(scroller.scrollTop, 200);
}, "Snaps to the positions adjusted by scroll-padding");
</script>