css/css-scroll-snap/scroll-margin.html

<!DOCTYPE html>
<link rel="help" href="https://drafts.csswg.org/css-scroll-snap-1/#scroll-margin" />
<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;
}
#another-target {
  width: 300px;
  height: 300px;
  top: 400px;
  left: 400px;
  background-color: blue;
  scroll-snap-align: start;
}
</style>

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

<script>
test(() => {
  target.style.scrollSnapAlign = "start";
  target.style.scrollMargin = "100px";
  target.style.left = "300px";
  target.style.top = "300px";

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

  target.style.scrollSnapAlign = "end";

  // `target position (300px, 300px)` + `target size (300px, 300px) +
  // `margin (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-margin");

test(() => {
  target.style.left = "0px";
  target.style.top = "0px";
  target.style.scrollSnapAlign = "start";

  // Since the target is at (0px, 0px) in the scroll port, the added margin
  // should not be considered, and the snap points for this snap area should be
  // the closest points in the scroll port (i.e x=0 or y=0).
  target.style.scrollMargin = "200px";

  // Distance from target without margin:
  // `scroll position (150px, 150px)` - `target position (0px, 0px)`
  // = (150px, 150px)
  //
  // Distance from target with margin:
  // `scroll position (150px, 150px)` - [`target position (0px, 0px)` -
  // `target margin (200px, 200px)`]
  // = (350px, 350px)
  //
  // Distance from other target:
  // `other target position (400px, 400px)` - `scroll position (150px, 150px)`
  // = (250px, 250px)
  //
  // Therefore if the "out-of-scrollport" scroll-margin contributes to the
  // calculation, then the other target would be snapped to. However if the
  // scroll-margin is not considered, then the (0px, 0px) target should be
  // snapped to.
  scroller.scrollTo(150, 150);
  assert_equals(scroller.scrollLeft, 0);
  assert_equals(scroller.scrollTop, 0);
}, "scroll-margin doesn't contribute to the snap position of the element " +
   "if it's outside of the scroll port");
</script>