test_bug1831148.html

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<!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>

<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1831148">Mozilla Bug 1831148</a>

<div id="marker"></div>

<div id="outer">

  <div id="inner"></div>

</div>

<script>

SimpleTest.waitForExplicitFinish();

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;

    // Floating point value diverges from scrollTop.

    isfuzzy(bcrTop, outer.scrollTop, 1, "scrollTop and BCR top match");

  SimpleTest.finish();

});

</script>