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<meta charset="utf-8">
<title>Test for Bug 1490890</title>
<script src="/tests/SimpleTest/SimpleTest.js"></script>
<link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
#flex {
display: flex;
flex-direction: column;
height: 100px;
max-height: 100px;
overflow: hidden;
border: 1px solid black;
#overflowAuto {
overflow: auto;
white-space: pre-wrap;
<div id="display">
<div id="content">
<div id="flex">
<div id="overflowAuto">
<!-- Populated by test JS below: -->
<div id="tall"></div>
<div id="testNode">abc</div>
<pre id="test">
<script type="application/javascript">
"use strict";
/** Test for Bug 1490890 **/
* This test checks how many reflows are required, when we make a change inside
* a flex item, with a tall scrollable sibling flex item.
const gUtils = SpecialPowers.getDOMWindowUtils(window);
// The elements that we will modify here:
const gTall = document.getElementById("tall");
const gTestNode = document.getElementById("testNode");
// Helper function to undo our modifications:
function cleanup()
gTall.firstChild.remove(); = "";
// Flush layout & return the global frame-reflow-count
function getReflowCount()
let unusedVal = document.getElementById("flex").offsetHeight; // flush layout
return gUtils.framesReflowed;
// This function changes gTestNode to "display:none", and returns the number
// of frames that need to be reflowed as a result of that tweak.
function makeTweakAndCountReflows()
let beforeCount = getReflowCount(); = "none";
let afterCount = getReflowCount();
let numReflows = afterCount - beforeCount;
if (numReflows <= 0) {
ok(false, "something's wrong -- we should've reflowed *something*");
return numReflows;
// -----------------------------
const testLineCount = 100;
const refLineCount = 5000;
// "Reference" measurement: put enough lines of text into gTall to trigger
// a vertical scrollbar, and then see how many frames need to be reflowed
// in response to a tweak in gTestNode:
let text = document.createTextNode("a\n".repeat(testLineCount));
let numReferenceReflows = makeTweakAndCountReflows();
// "Test" measurement: put many more lines of text into gTall (many more than
// for reference case), and then see how many frames need to be reflowed
// in response to a tweak in gTestNode:
text = document.createTextNode("a\n".repeat(refLineCount));
let numTestReflows = makeTweakAndCountReflows();
is(numTestReflows, numReferenceReflows,
"Tweak should trigger the same number of reflows regardless of " +
"how much content is present in descendant of sibling");