position: sticky does not treat the element's borders as part of it

[AtkinsSJ]

With this reduced case:

<style>
html, body {
    margin: 0;
}
.sticky {
    position: sticky;
    top: 0;
    left: 0;
    right: 0;
    background-color: cyan;
    border-top: 20px solid magenta;
    padding: 0.5em;
}
</style>
<div style="height: 10000px">
    <div class="sticky">I'm sticky</div>
</div>

When scrolling down, other browsers keep the full border in view, but we do not.