css/css-scroll-snap/snap-to-combination-of-two-elements-1.html

<!DOCTYPE html>
<meta name="viewport" content="width=device-width">
<title>
  Snap to points of combinations of two different elements
</title>
<link rel="help" href="https://drafts.csswg.org/css-scroll-snap-1/"/>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
div {
  position: absolute;
  margin: 0px;
}

#scroller {
  height: 600px;
  width: 600px;
  overflow: hidden;
  scroll-snap-type: both mandatory;
}

#space {
  width: 2000px;
  height: 2000px;
}

#left-top {
  /*
   * the scroll position of this `scroll-snap-align: start start` is (200, 200)
   */
  left: 200px;
  top: 200px;
  height: 450px;
  width: 450px;
  background-color: rgb(255, 0, 0);
  opacity: 0.5;
  scroll-snap-align: start start;
}

#right-bottom {
  /*
   * the scroll position of this `scroll-snap-align: end end` is (50, 50),
   * i.e, (`left` - scroll container's `width`  + this element's `width,
           `top`  - scroll container's `height` + this element's `height)
   */
  left: 600px;
  top: 600px;
  width: 50px;
  height: 50px;
  background-color: rgb(0, 255, 0);
  opacity: 0.5;
  scroll-snap-align: end end;
}

</style>
<div id="scroller">
  <div id="space"></div>
  <div id="left-top"></div>
  <div id="right-bottom"></div>
</div>
<script>
test(t => {
  const scroller = document.getElementById("scroller");
  assert_equals(scroller.scrollLeft, 50);
  assert_equals(scroller.scrollTop, 50);

  scroller.scrollTo(100, 150);
  assert_equals(scroller.scrollLeft, 50);
  assert_equals(scroller.scrollTop, 200);

  scroller.scrollTo(300, 300);
  assert_equals(scroller.scrollLeft, 200);
  assert_equals(scroller.scrollTop, 200);

  scroller.scrollTo(150, 100);
  assert_equals(scroller.scrollLeft, 200);
  assert_equals(scroller.scrollTop, 50);
}, 'Snap to points of combinations of two different elements');
</script>