diff -r 000000000000 -r 6474c204b198 browser/base/content/test/general/browser_tabopen_reflows.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/base/content/test/general/browser_tabopen_reflows.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,129 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+XPCOMUtils.defineLazyGetter(this, "docShell", () => {
+  return window.QueryInterface(Ci.nsIInterfaceRequestor)
+               .getInterface(Ci.nsIWebNavigation)
+               .QueryInterface(Ci.nsIDocShell);
+});
+
+const EXPECTED_REFLOWS = [
+  // tabbrowser.adjustTabstrip() call after tabopen animation has finished
+  "adjustTabstrip@chrome://browser/content/tabbrowser.xml|" +
+    "_handleNewTab@chrome://browser/content/tabbrowser.xml|" +
+    "onxbltransitionend@chrome://browser/content/tabbrowser.xml|",
+
+  // switching focus in updateCurrentBrowser() causes reflows
+  "updateCurrentBrowser@chrome://browser/content/tabbrowser.xml|" +
+    "onselect@chrome://browser/content/browser.xul|",
+
+  // switching focus in openLinkIn() causes reflows
+  "openLinkIn@chrome://browser/content/utilityOverlay.js|" +
+    "openUILinkIn@chrome://browser/content/utilityOverlay.js|" +
+    "BrowserOpenTab@chrome://browser/content/browser.js|",
+
+  // accessing element.scrollPosition in _fillTrailingGap() flushes layout
+  "get_scrollPosition@chrome://global/content/bindings/scrollbox.xml|" +
+    "_fillTrailingGap@chrome://browser/content/tabbrowser.xml|" +
+    "_handleNewTab@chrome://browser/content/tabbrowser.xml|" +
+    "onxbltransitionend@chrome://browser/content/tabbrowser.xml|",
+
+  // The TabView iframe causes reflows in the parent document.
+  "iQClass_height@chrome://browser/content/tabview.js|" +
+    "GroupItem_getContentBounds@chrome://browser/content/tabview.js|" +
+    "GroupItem_shouldStack@chrome://browser/content/tabview.js|" +
+    "GroupItem_arrange@chrome://browser/content/tabview.js|" +
+    "GroupItem_add@chrome://browser/content/tabview.js|" +
+    "GroupItems_newTab@chrome://browser/content/tabview.js|" +
+    "TabItem__reconnect@chrome://browser/content/tabview.js|" +
+    "TabItem@chrome://browser/content/tabview.js|" +
+    "TabItems_link@chrome://browser/content/tabview.js|" +
+    "TabItems_init/this._eventListeners.open@chrome://browser/content/tabview.js|",
+
+  // SessionStore.getWindowDimensions()
+  "ssi_getWindowDimension@resource:///modules/sessionstore/SessionStore.jsm|" +
+    "ssi_updateWindowFeatures/<@resource:///modules/sessionstore/SessionStore.jsm|" +
+    "ssi_updateWindowFeatures@resource:///modules/sessionstore/SessionStore.jsm|" +
+    "ssi_collectWindowData@resource:///modules/sessionstore/SessionStore.jsm|",
+
+  // tabPreviews.capture()
+  "tabPreviews_capture@chrome://browser/content/browser.js|" +
+    "tabPreviews_handleEvent/<@chrome://browser/content/browser.js|",
+
+  // tabPreviews.capture()
+  "tabPreviews_capture@chrome://browser/content/browser.js|" +
+    "@chrome://browser/content/browser.js|"
+];
+
+const PREF_PRELOAD = "browser.newtab.preload";
+const PREF_NEWTAB_DIRECTORYSOURCE = "browser.newtabpage.directorySource";
+
+/*
+ * This test ensures that there are no unexpected
+ * uninterruptible reflows when opening new tabs.
+ */
+function test() {
+  waitForExplicitFinish();
+
+  Services.prefs.setBoolPref(PREF_PRELOAD, false);
+  Services.prefs.setCharPref(PREF_NEWTAB_DIRECTORYSOURCE, "data:application/json,{}");
+  registerCleanupFunction(() => {
+    Services.prefs.clearUserPref(PREF_PRELOAD);
+    Services.prefs.clearUserPref(PREF_NEWTAB_DIRECTORYSOURCE);
+  });
+
+  // Add a reflow observer and open a new tab.
+  docShell.addWeakReflowObserver(observer);
+  BrowserOpenTab();
+
+  // Wait until the tabopen animation has finished.
+  waitForTransitionEnd(function () {
+    // Remove reflow observer and clean up.
+    docShell.removeWeakReflowObserver(observer);
+    gBrowser.removeCurrentTab();
+
+    finish();
+  });
+}
+
+let observer = {
+  reflow: function (start, end) {
+    // Gather information about the current code path.
+    let path = (new Error().stack).split("\n").slice(1).map(line => {
+      return line.replace(/:\d+:\d+$/, "");
+    }).join("|");
+    let pathWithLineNumbers = (new Error().stack).split("\n").slice(1).join("|");
+
+    // Stack trace is empty. Reflow was triggered by native code.
+    if (path === "") {
+      return;
+    }
+
+    // Check if this is an expected reflow.
+    for (let stack of EXPECTED_REFLOWS) {
+      if (path.startsWith(stack)) {
+        ok(true, "expected uninterruptible reflow '" + stack + "'");
+        return;
+      }
+    }
+
+    ok(false, "unexpected uninterruptible reflow '" + pathWithLineNumbers + "'");
+  },
+
+  reflowInterruptible: function (start, end) {
+    // We're not interested in interruptible reflows.
+  },
+
+  QueryInterface: XPCOMUtils.generateQI([Ci.nsIReflowObserver,
+                                         Ci.nsISupportsWeakReference])
+};
+
+function waitForTransitionEnd(callback) {
+  let tab = gBrowser.selectedTab;
+  tab.addEventListener("transitionend", function onEnd(event) {
+    if (event.propertyName === "max-width") {
+      tab.removeEventListener("transitionend", onEnd);
+      executeSoon(callback);
+    }
+  });
+}