1 files changed, 58 insertions, 0 deletions
diff --git a/browser/base/content/test/performance/browser_windowclose.js b/browser/base/content/test/performance/browser_windowclose.js
new file mode 100644
index 0000000000..26eecf9539
--- /dev/null
+++ b/browser/base/content/test/performance/browser_windowclose.js
@@ -0,0 +1,58 @@
+"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 windows. When the
+ * window is closed, the test waits until the original window
+ * has activated.
+ */
+add_task(async function () {
+  // Ensure that this browser window starts focused. This seems to be
+  // necessary to avoid intermittent failures when running this test
+  // on repeat.
+  await new Promise(resolve => {
+    waitForFocus(resolve, window);
+  });
+
+  let win = await BrowserTestUtils.openNewBrowserWindow();
+  await new Promise(resolve => {
+    waitForFocus(resolve, win);
+  });
+
+  // At the time of writing, there are no reflows on window closing.
+  // 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 for window close."
+  );
+
+  await withPerfObserver(
+    async function () {
+      let promiseOrigBrowserFocused = TestUtils.waitForCondition(() => {
+        return Services.focus.activeWindow == window;
+      });
+      await BrowserTestUtils.closeWindow(win);
+      await promiseOrigBrowserFocused;
+    },
+    {
+      expectedReflows: EXPECTED_REFLOWS,
+    },
+    win
+  );
+});