1 files changed, 91 insertions, 0 deletions
diff --git a/browser/base/content/test/performance/browser_tabclose_grow.js b/browser/base/content/test/performance/browser_tabclose_grow.js
new file mode 100644
index 0000000000..7ad43809cd
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+++ b/browser/base/content/test/performance/browser_tabclose_grow.js
@@ -0,0 +1,91 @@
+"use strict";
+
+/**
+ * WHOA THERE: We should never be adding new things to EXPECTED_REFLOWS.
+ * Instead of adding reflows to the list, you should be modifying your code to
+ * avoid the reflow.
+ *
+ * See https://firefox-source-docs.mozilla.org/performance/bestpractices.html
+ * 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 () {
+  // Force-enable tab animations
+  gReduceMotionOverride = false;
+
+  await ensureNoPreloadedBrowser();
+  await disableFxaBadge();
+
+  // The test starts on about:blank and opens an about:blank
+  // tab which triggers opening the toolbar since
+  // ensureNoPreloadedBrowser sets AboutNewTab.newTabURL to about:blank.
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.toolbars.bookmarks.visibility", "never"]],
+  });
+
+  // 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();
+
+  function isInTabStrip(r) {
+    return (
+      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 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, "TabAnimationEnd");
+      await switchDone;
+    },
+    {
+      expectedReflows: EXPECTED_REFLOWS,
+      frames: {
+        filter: rects => rects.filter(r => !isInTabStrip(r)),
+      },
+    }
+  );
+
+  await removeAllButFirstTab();
+});