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<!DOCTYPE html>
<link rel="help" href="https://drafts.csswg.org/css-scroll-anchoring-1/">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
body {
height: 200vh;
}
#anchor {
width: 100px;
height: 100px;
background-color: blue;
}
</style>
<div id="expander"></div>
<div id="anchor"></div>
<script>
// This tests that scroll anchor adjustments can happen by quantities smaller
// than a device pixel.
//
// Unfortunately, we can't test this by simply reading 'scrollTop', because
// 'scrollTop' may be rounded to the nearest CSS pixel. So, to test that
// subpixel adjustments can in fact happen, we repeatedly trigger a scroll
// adjustment in a way that would produce a different final .scrollTop value,
// depending on whether or not we rounded each adjustment as we apply it.
test(() => {
let scroller = document.scrollingElement;
let expander = document.querySelector("#expander");
let anchor = document.querySelector("#anchor");
const initialTop = 10;
// Scroll 10px to activate scroll anchoring
scroller.scrollTop = initialTop;
// Helper to insert a div with specified height before the anchor node
function addChild(height) {
let child = document.createElement("div");
child.style.height = `${height}px`;
anchor.before(child);
}
// Calculate what fraction of a CSS pixel corresponds to one device pixel
let devicePixel = 1.0 / window.devicePixelRatio;
assert_true(devicePixel <= 1.0, "there should be more device pixels than CSS pixels");
// The 0.5 is an arbitrary scale when creating the subpixel delta
let delta = 0.5 * devicePixel;
// To help us check for for premature rounding of adjustments, we'll
// trigger "count" subpixel adjustments of size "delta", where "count" is
// the first positive integer such that:
// round(count * delta) != count * round(delta)
// As round(X) and count are integers, this happens when:
// count * delta = count * round(delta) +/- 1
// Solving for count:
// count = 1 / abs(delta - round(delta))
// Note that we don't need to worry about the denominator being zero, as:
// 0 < devicePixel <= 1
// And so halving devicePixel should never yield a whole number.
let count = 1 / Math.abs(delta - Math.round(delta));
for (let i = 0; i < count; i++) {
addChild(delta);
// Trigger an anchor adjustment by forcing a layout flush
scroller.scrollTop;
}
let destination = Math.round(initialTop + delta * count);
assert_equals(scroller.scrollTop, destination,
`adjusting by ${delta}px, ${count} times, should be the same as adjusting by ${delta * count}px, once.`);
}, "Test that scroll anchor adjustments can happen by a sub device-pixel amount.");
</script>
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