fune/browser/base/content/test/performance/browser_tabclose_grow.js

"use strict";

/**
 * WHOA THERE: We should never be adding new things to EXPECTED_REFLOWS. This
 * is a whitelist that should slowly go away as we improve the performance of
 * the front-end. Instead of adding more reflows to the whitelist, you should
 * be modifying your code to avoid the reflow.
 *
 * See https://developer.mozilla.org/en-US/Firefox/Performance_best_practices_for_Firefox_fe_engineers
 * for tips on how to do that.
 */
const EXPECTED_REFLOWS = [
  /**
   * Nothing here! Please don't add anything new!
   */
];

/*
 * This test ensures that there are no unexpected
 * uninterruptible reflows when closing a tab that will
 * cause the existing tabs to grow bigger.
 */
add_task(async function() {
  await ensureNoPreloadedBrowser();

  // At the time of writing, there are no reflows on tab closing with
  // tab growth. Mochitest will fail if we have no assertions, so we
  // add one here to make sure nobody adds any new ones.
  Assert.equal(EXPECTED_REFLOWS.length, 0,
    "We shouldn't have added any new expected reflows.");

  // Compute the number of tabs we can put into the strip without
  // overflowing. If we remove one of the tabs, we know that the
  // remaining tabs will grow to fill the remaining space in the
  // tabstrip.
  const TAB_COUNT_FOR_GROWTH = computeMaxTabCount();
  await createTabs(TAB_COUNT_FOR_GROWTH);

  let lastTab = gBrowser.tabs[gBrowser.tabs.length - 1];
  await BrowserTestUtils.switchTab(gBrowser, lastTab);

  let tabStripRect = gBrowser.tabContainer.arrowScrollbox.getBoundingClientRect();

  await withPerfObserver(async function() {
    let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
    let tab = gBrowser.tabs[gBrowser.tabs.length - 1];
    gBrowser.removeTab(tab, { animate: true });
    await BrowserTestUtils.waitForEvent(tab, "transitionend",
      false, e => e.propertyName === "max-width");
    await switchDone;
  }, {expectedReflows: EXPECTED_REFLOWS,
      frames: {
        filter: rects => rects.filter(r => !(
          // We expect plenty of changed rects within the tab strip.
          r.y1 >= tabStripRect.top && r.y2 <= tabStripRect.bottom &&
          r.x1 >= tabStripRect.left && r.x2 <= tabStripRect.right &&
          // It would make sense for each rect to have a width smaller than
          // a tab (ie. tabstrip.width / tabcount), but tabs are small enough
          // that they sometimes get reported in the same rect.
          // So we accept up to the width of n-1 tabs.
          r.w <= (gBrowser.tabs.length - 1) * Math.ceil(tabStripRect.width / gBrowser.tabs.length)
        ))
      }
     });

  await removeAllButFirstTab();
});