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Test Info:

<!DOCTYPE html>
<title>
Snap to an area where the element's scroll-margin is visible but not the
element itself (using y-axis snap type)
</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
div {
position: absolute;
margin: 0px;
}
#scroller {
height: 600px;
width: 600px;
overflow: scroll;
scroll-snap-type: y mandatory;
}
#space {
width: 2000px;
height: 2000px;
}
.snap {
width: 200px;
height: 200px;
background-color: blue;
scroll-snap-align: start;
}
#top-visible {
left: 0px;
top: 0px;
}
#bottom-visible {
left: 0px;
top: 800px;
}
#middle-margin-visible {
left: 800px;
top: 700px;
/* 300px makes snap area visible with margin but non-visible without it */
scroll-margin-left: 300px;
}
</style>
<div id="scroller">
<div id="space"></div>
<div id="top-visible" class="snap"></div>
<div id="middle-margin-visible" class="snap"></div>
<div id="bottom-visible" class="snap"></div>
</div>
<script>
test(() => {
const scroller = document.getElementById("scroller");
scroller.scrollTo(0, 0);
assert_equals(scroller.scrollLeft, 0);
assert_equals(scroller.scrollTop, 0);
scroller.scrollTo(0, 500);
assert_equals(scroller.scrollLeft, 0);
assert_equals(scroller.scrollTop, 700);
}, 'Scroll margin should be considered when calculating snap area visibilty \
while snapping on the y-axis');
</script>