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Test Info:

<!DOCTYPE html>
<html>
<script src="/tests/SimpleTest/SimpleTest.js"></script>
<link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
<style>
#marker {
width: 1px;
height: 1px;
}
#outer {
height: 1000px;
background: #a0ffff;
overflow: scroll;
}
#inner {
height: 16593200px;
background: #ffa0a0;
}
.log {
white-space: pre;
}
</style>
<div id="marker"></div>
<div id="outer">
<div id="inner"></div>
</div>
<script>
SimpleTest.waitForExplicitFinish();
// getBoundingClientRect() returns coordinates that are stored internally as
// 32-bit floats (CSSPoint), whereas scrollTop is reported as a double. At the
// multi-million-pixel scroll offsets exercised here, one float ULP exceeds a
// CSS pixel, so the two unavoidably diverge by an amount that grows with the
// magnitude. Returns one float ULP at the given magnitude (23-bit mantissa).
function floatEpsilon(value) {
const magnitude = Math.abs(value);
if (magnitude === 0) {
return 0;
}
return Math.pow(2, Math.floor(Math.log2(magnitude)) - 23);
}
addLoadEvent(() => {
const interval = 20;
const increment = outer.scrollHeight / interval;
const offset = marker.getBoundingClientRect().bottom;
for (let i = 0; i < interval; i++) {
outer.scrollTop = i * increment;
console.log(outer.scrollTop);
// Shift to account for viewport coordinate shift.
const bcrTop = -inner.getBoundingClientRect().top + offset;
// Tolerate device-pixel snapping plus the float quantization of the BCR
// (a few ULPs across the negate/offset arithmetic), which scales with the
// scroll offset.
const epsilon = devicePixelRatio + 2 * floatEpsilon(outer.scrollTop);
isfuzzy(bcrTop, outer.scrollTop, epsilon, "scrollTop and BCR top match");
}
SimpleTest.finish();
});
</script>