author Sandor Molnar <>
Fri, 24 Sep 2021 00:43:42 +0300
changeset 593102 4eda9eb8926bdd50f4b80128ce3475eb7c6d9a4d
parent 469640 c9c0c6f2eed54a187e124942e53c3660b4cf17d8
permissions -rw-r--r--
Merge autoland to mozilla-central. a=merge

  <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;
<a target="_blank" href="">Mozilla Bug 1490890</a>
<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");