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Test Info:

<!DOCTYPE html>
<title>
Snap to a visible area only even when there is a closer snap point for an area
that is closer but not visible (using both axes snap type), where the relevant
snap areas are pseudo-elements
</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
body, html { height: 100%; }
div {
position: absolute;
margin: 0px;
}
#scroller {
height: 600px;
width: 600px;
overflow: scroll;
scroll-snap-type: both mandatory;
}
#space {
width: 2000px;
height: 2000px;
}
.snap {
width: 200px;
height: 200px;
background-color: blue;
scroll-snap-align: start;
}
#left-top {
left: 0px;
top: 0px;
}
#left-top::before {
position: absolute;
margin: 0px;
content: "";
display:block;
left: 0px;
top: 800px;
width: 200px;
height: 200px;
background-color: yellow;
scroll-snap-align: start;
}
#left-top::after {
position: absolute;
margin: 0px;
content: "";
display:block;
left: 800px;
top: 0px;
width: 200px;
height: 200px;
background-color: yellow;
scroll-snap-align: start;
}
</style>
<div id="scroller">
<div id="space"></div>
<div id="left-top" class="snap"></div>
</div>
<script>
test(t => {
const scroller = document.getElementById("scroller");
scroller.scrollTo(0, 0);
assert_equals(scroller.scrollLeft, 0);
assert_equals(scroller.scrollTop, 0);
scroller.scrollTo(500, 600);
assert_equals(scroller.scrollLeft, 0);
assert_equals(scroller.scrollTop, 800);
scroller.scrollTo(600, 500);
assert_equals(scroller.scrollLeft, 800);
assert_equals(scroller.scrollTop, 0);
}, 'Only snap to visible areas in the case where taking the closest snap point of \
each axis does not snap to a visible area');
</script>