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<!doctype html>
<title>Test for bug 1579929</title>
<link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css">
<script src="/tests/SimpleTest/SimpleTest.js"></script>
<script src="/tests/SimpleTest/EventUtils.js"></script>
.container {
display: flex;
flex-direction: column;
overflow-y: scroll;
#outer {
width: 500px;
height: 300px;
<div class="container" id="outer">
<div class="item" id="firstItem" style="width: 50px">
<!--popuplated by script-->
<div class="container">
<div class="container">
<div class="container">
<div class="container">
<div class="container">
<div class="item">Item</div>
// Globals/constants:
const gUtils = SpecialPowers.DOMWindowUtils;
// This needs to match/exceed nsPresContext::sInterruptChecksToSkip in order
// for us to actually be able to trigger the mHasPendingInterrupt=true codepath
// in this test. Each of these "dummy" blocks will trigger a call to
// nsPresContext::CheckForInterrupt, and after 200, we'll actually trigger an
// interrupt.
const gInterruptCheckThreshold = 200;
// Expected to match the inline style="width:..." for #firstItem (we slowly
// increment it to trigger reflows):
let gFirstItemWidthPX = 50;
function main() {
const outer = document.getElementById("outer");
const firstItem = document.getElementById("firstItem");
// Insert a bunch of elements to be sure we actually get interrupted.
// (See description of gInterruptCheckThreshold above)
for (let i = 0; i < gInterruptCheckThreshold; i++) {
let dummyBlock = document.createElement("div");
dummyBlock.innerText = "dummy";
// Flush any pending relayout:
// Take control of the refresh driver
// Force reflow and store the "cost" (in num-frames-reflowed)
const costOfNormalReflow = forceReflow();
// Sanity-check: do that again and remeasure cost, to be sure it's stable:
const costOfNormalReflow2 = forceReflow();
is(costOfNormalReflow, costOfNormalReflow2,
"Our forceReflow function is expected to reliably trigger " +
"the same set of frames to be reflowed. If this fails, there's a " +
"bug in the test, or non-determinism in layout code.");
// Now, we force the next reflow to get interrupted, and then measure the
// cost under those conditions:
const costOfInterruptedReflow = forceReflow();
// Hopefully the interrupted one was less expensive...
ok(costOfInterruptedReflow <= costOfNormalReflow,
"num frames reflowed in interrupted reflow (" +
costOfInterruptedReflow +
") shouldn't exceed the num frames reflowed in normal reflow (" +
costOfNormalReflow + ")");
// This function dirties firstItem's width, forces a reflow, and
// returns the number of frames that were reflowed as a result.
function forceReflow() {
gFirstItemWidthPX++; = gFirstItemWidthPX + "px";
const origFramesReflowed = gUtils.framesReflowed;
return gUtils.framesReflowed - origFramesReflowed;