1 files changed, 100 insertions, 0 deletions
diff --git a/browser/base/content/test/performance/browser_tabopen_squeeze.js b/browser/base/content/test/performance/browser_tabopen_squeeze.js
new file mode 100644
index 0000000000..f92bdc2ea4
--- /dev/null
+++ b/browser/base/content/test/performance/browser_tabopen_squeeze.js
@@ -0,0 +1,100 @@
+"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 opening a new tab that will
+ * cause the existing tabs to squeeze smaller.
+ */
+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"]],
+  });
+
+  // Compute the number of tabs we can put into the strip without
+  // overflowing, and remove one, so that we can create
+  // TAB_COUNT_FOR_SQUEEE tabs, and then one more, which should
+  // cause the tab to squeeze to a smaller size rather than overflow.
+  const TAB_COUNT_FOR_SQUEEZE = computeMaxTabCount() - 1;
+
+  await createTabs(TAB_COUNT_FOR_SQUEEZE);
+
+  gURLBar.focus();
+
+  let tabStripRect =
+    gBrowser.tabContainer.arrowScrollbox.getBoundingClientRect();
+  let textBoxRect = gURLBar
+    .querySelector("moz-input-box")
+    .getBoundingClientRect();
+
+  await withPerfObserver(
+    async function () {
+      let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
+      BrowserOpenTab();
+      await BrowserTestUtils.waitForEvent(
+        gBrowser.selectedTab,
+        "TabAnimationEnd"
+      );
+      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)
+                )
+              )
+          ),
+        exceptions: [
+          {
+            name: "the urlbar placeolder moves up and down by a few pixels",
+            condition: r =>
+              r.x1 >= textBoxRect.left &&
+              r.x2 <= textBoxRect.right &&
+              r.y1 >= textBoxRect.top &&
+              r.y2 <= textBoxRect.bottom,
+          },
+        ],
+      },
+    }
+  );
+
+  await removeAllButFirstTab();
+});