Bug 2062317, be more synchronous with DOMContentLoaded, r=farre

Differential Revision: https://phabricator.services.mozilla.com/D321732

docshell/base/nsDocShell.cpp

@@ -10342,7 +10342,7 @@ nsresult nsDocShell::CompleteInitialAboutBlankLoad(
 
   // Mechanisms in Document will force a load from EndLoad()
   // even if there are still blockers.
-  doc->EndLoad();
+  doc->EndLoad(/* aFireDOMContentLoadedSync = */ true);
   // Can't assert any postcondition, because the load event
   // handler may have started loading something new in this
   // docshell.

dom/base/Document.cpp

@@ -8770,7 +8770,7 @@ void Document::MozSetImageElement(const nsAString& aImageElementId,
   }
 }
 
-void Document::DispatchContentLoadedEvents() {
+void Document::DispatchContentLoadedEvents(bool aFinishSync) {
   // If you add early returns from this method, make sure you're
   // calling UnblockOnload properly.
 
@@ -8882,6 +8882,19 @@ void Document::DispatchContentLoadedEvents() {
     }
   }
 
+  if (aFinishSync) {
+    FinishDOMContentLoaded();
+    return;
+  }
+
+  // Keep the load event on a task, so that its timing does not change.
+  nsCOMPtr<nsIRunnable> ev =
+      NewRunnableMethod("Document::FinishDOMContentLoaded", this,
+                        &Document::FinishDOMContentLoaded);
+  Dispatch(ev.forget());
+}
+
+void Document::FinishDOMContentLoaded() {
   if (mSetCompleteAfterDOMContentLoaded) {
     SetReadyStateInternal(ReadyState::READYSTATE_COMPLETE);
     mSetCompleteAfterDOMContentLoaded = false;
@@ -8890,7 +8903,7 @@ void Document::DispatchContentLoadedEvents() {
   UnblockOnload(true);
 }
 
-void Document::EndLoad() {
+void Document::EndLoad(bool aFireDOMContentLoadedSync) {
   bool turnOnEditing =
       mParser && (IsInDesignMode() || mContentEditableCount > 0);
 
@@ -8938,7 +8951,7 @@ void Document::EndLoad() {
   }
   mDidCallBeginLoad = false;
 
-  UnblockDOMContentLoaded();
+  UnblockDOMContentLoaded(aFireDOMContentLoadedSync);
 
   if (turnOnEditing) {
     EditingStateChanged();
@@ -8963,7 +8976,7 @@ void Document::EndLoad() {
   }
 }
 
-void Document::UnblockDOMContentLoaded() {
+void Document::UnblockDOMContentLoaded(bool aFireSync) {
   MOZ_ASSERT(mBlockDOMContentLoaded);
   if (--mBlockDOMContentLoaded != 0 || mDidFireDOMContentLoaded) {
     return;
@@ -8974,17 +8987,28 @@ void Document::UnblockDOMContentLoaded() {
 
   mDidFireDOMContentLoaded = true;
 
+  MOZ_RELEASE_ASSERT(NS_IsMainThread());
   MOZ_ASSERT(IsInitialDocument() || mReadyState == READYSTATE_INTERACTIVE);
-  if (!mSynchronousDOMContentLoaded) {
-    MOZ_RELEASE_ASSERT(NS_IsMainThread());
-    MOZ_ASSERT(!IsInitialDocument());
-    nsCOMPtr<nsIRunnable> ev =
-        NewRunnableMethod("Document::DispatchContentLoadedEvents", this,
-                          &Document::DispatchContentLoadedEvents);
-    Dispatch(ev.forget());
-  } else {
-    DispatchContentLoadedEvents();
+
+  // These documents need the load event unblocked before we return.
+  if (mSynchronousDOMContentLoaded) {
+    MOZ_ASSERT(nsContentUtils::IsSafeToRunScript());
+    DispatchContentLoadedEvents(/* aFinishSync = */ true);
+    return;
   }
+
+  if (aFireSync &&
+      StaticPrefs::dom_document_domcontentloaded_synchronous_enabled()) {
+    nsContentUtils::AddScriptRunner(
+        NewRunnableMethod<bool>("Document::DispatchContentLoadedEvents", this,
+                                &Document::DispatchContentLoadedEvents, false));
+    return;
+  }
+
+  MOZ_ASSERT(!IsInitialDocument());
+  Dispatch(NewRunnableMethod<bool>("Document::DispatchContentLoadedEvents",
+                                   this, &Document::DispatchContentLoadedEvents,
+                                   true));
 }
 
 void Document::ElementStateChanged(Element* aElement, ElementState aStateMask) {
@@ -15326,7 +15350,8 @@ class UnblockParsingPromiseHandler final : public PromiseNativeHandler {
       // parser state for this document.  Maybe someone caused it to stop being
       // parsed, so CreatorParserOrNull() is returning null, but we still want
       // to unblock these.
-      mDocument->UnblockDOMContentLoaded();
+      // Async, because this also runs from our destructor.
+      mDocument->UnblockDOMContentLoaded(/* aFireSync = */ false);
       mDocument->UnblockOnload(false);
     }
     mParser = nullptr;

dom/base/Document.h

@@ -1670,7 +1670,9 @@ class Document : public nsINode,
                          NotNull<const Encoding*>& aEncoding,
                          nsHtml5TreeOpExecutor* aExecutor);
 
-  MOZ_CAN_RUN_SCRIPT void DispatchContentLoadedEvents();
+  MOZ_CAN_RUN_SCRIPT void DispatchContentLoadedEvents(bool aFinishSync);
+  // Unblocks the load event. An aborted load also gets readyState complete.
+  MOZ_CAN_RUN_SCRIPT void FinishDOMContentLoaded();
 
   // TODO: Convert this to MOZ_CAN_RUN_SCRIPT (bug 1415230)
   MOZ_CAN_RUN_SCRIPT_BOUNDARY void DispatchPageTransition(
@@ -2240,7 +2242,10 @@ class Document : public nsINode,
   uint32_t UpdateNestingLevel() { return mUpdateNestLevel; }
 
   void BeginLoad();
-  virtual void EndLoad();
+  // aFireDOMContentLoadedSync must be false for a terminated parse.
+  // See bug 344305.
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY virtual void EndLoad(
+      bool aFireDOMContentLoadedSync);
 
   enum ReadyState {
     READYSTATE_UNINITIALIZED = 0,
@@ -2643,7 +2648,8 @@ class Document : public nsINode,
 
   void BlockDOMContentLoaded() { ++mBlockDOMContentLoaded; }
 
-  MOZ_CAN_RUN_SCRIPT_BOUNDARY void UnblockDOMContentLoaded();
+  // aFireSync false fires DOMContentLoaded from a task. See bug 344305.
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY void UnblockDOMContentLoaded(bool aFireSync);
 
   /**
    * Notification that the page has been shown, for documents which are loaded

dom/base/test/file_domcontentloaded_document_open.html

@@ -0,0 +1,18 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+<meta charset="utf-8">
+<script>
+document.addEventListener("DOMContentLoaded", () => {
+  // document.open() clears the content while DOMContentLoaded is being fired.
+  document.open();
+  document.write(
+    "<!DOCTYPE html><html><body><script>" +
+    "parent.postMessage('reopened', '*');" +
+    "<\/script></body></html>");
+  document.close();
+});
+</script>
+</head>
+<body></body>
+</html>

dom/base/test/file_domcontentloaded_terminated_parse.html

@@ -0,0 +1,13 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+<meta charset="utf-8">
+<script>
+document.addEventListener("DOMContentLoaded",
+                          () => parent.postMessage("DOMContentLoaded", "*"));
+// Terminate the parse, see bug 344305.
+window.stop();
+</script>
+</head>
+<body></body>
+</html>

dom/base/test/mochitest.toml

@@ -12,6 +12,8 @@ support-files = [
   "iframe_main_bug1022229.html",
   "iframe_sandbox_bug1022229.html",
   "file_empty.html",
+  "file_domcontentloaded_document_open.html",
+  "file_domcontentloaded_terminated_parse.html",
   "iframe_postMessage_solidus.html",
   "file_setname.html",
   "345339_iframe.html",
@@ -352,6 +354,10 @@ skip-if = [
   "http3",
 ]
 
+["test_domcontentloaded_document_open.html"]
+
+["test_domcontentloaded_terminated_parse.html"]
+
 ["test_domparser_null_char.html"]
 
 ["test_domparsing.html"]

dom/base/test/test_domcontentloaded_document_open.html

@@ -0,0 +1,22 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+<title>Test document.open() from a DOMContentLoaded listener</title>
+<script src="/tests/SimpleTest/SimpleTest.js"></script>
+<link rel="stylesheet" href="/tests/SimpleTest/test.css">
+</head>
+<body>
+<iframe id="f"></iframe>
+<script>
+SimpleTest.waitForExplicitFinish();
+
+window.addEventListener("message", event => {
+  is(event.data, "reopened",
+     "the content written from a DOMContentLoaded listener was parsed");
+  SimpleTest.finish();
+});
+
+document.getElementById("f").src = "file_domcontentloaded_document_open.html";
+</script>
+</body>
+</html>

dom/base/test/test_domcontentloaded_terminated_parse.html

@@ -0,0 +1,22 @@
+<!DOCTYPE HTML>
+<html>
+<head>
+<title>Test DOMContentLoaded after a terminated parse</title>
+<script src="/tests/SimpleTest/SimpleTest.js"></script>
+<link rel="stylesheet" href="/tests/SimpleTest/test.css">
+</head>
+<body>
+<iframe id="f"></iframe>
+<script>
+SimpleTest.waitForExplicitFinish();
+
+window.addEventListener("message", event => {
+  is(event.data, "DOMContentLoaded",
+     "DOMContentLoaded fires after the parse was terminated");
+  SimpleTest.finish();
+});
+
+document.getElementById("f").src = "file_domcontentloaded_terminated_parse.html";
+</script>
+</body>
+</html>

dom/base/test/unit/test_blockParsing.js

@@ -24,6 +24,7 @@ const server = XPCShellContentUtils.createHttpServer({
 
 // XML document with only a <script> tag as the document element.
 const PAGE_URL = "http://example.com/";
+const DCL_PAGE_URL = "http://example.com/blockdcl";
 server.registerPathHandler("/", (request, response) => {
   response.setHeader("Content-Type", "application/xhtml+xml");
   response.write(String.raw`<!DOCTYPE html>
@@ -31,6 +32,11 @@ server.registerPathHandler("/", (request, response) => {
   `);
 });
 
+server.registerPathHandler("/blockdcl", (request, response) => {
+  response.setHeader("Content-Type", "text/html");
+  response.write("<!DOCTYPE html><html><body>blockdcl</body></html>");
+});
+
 let resolveResumeScriptPromise;
 let resumeScriptPromise = new Promise(resolve => {
   resolveResumeScriptPromise = resolve;
@@ -119,3 +125,57 @@ add_task(async function test_nested_blockParser() {
   let page = await pagePromise;
   await page.close();
 });
+
+// Tests that blockParsing() also blocks DOMContentLoaded, and that
+// DOMContentLoaded is fired once the blocker promise settles.
+add_task(async function test_blockParsing_blocksDOMContentLoaded() {
+  let resolveBlockerPromise;
+  let dclFired = false;
+  let dclPromise;
+
+  let docElementPromise = TestUtils.topicObserved(
+    "document-element-inserted",
+    doc => {
+      if (doc.location.href !== DCL_PAGE_URL) {
+        return false;
+      }
+
+      let blockerPromise = new Promise(resolve => {
+        resolveBlockerPromise = resolve;
+      });
+      doc.blockParsing(blockerPromise);
+
+      dclPromise = new Promise(resolve => {
+        doc.addEventListener(
+          "DOMContentLoaded",
+          () => {
+            dclFired = true;
+            resolve();
+          },
+          { once: true }
+        );
+      });
+      return true;
+    }
+  );
+
+  let pagePromise = XPCShellContentUtils.loadContentPage(DCL_PAGE_URL, {
+    remote: false,
+  });
+
+  await docElementPromise;
+
+  // Make some trips through the event loop to be safe.
+  await delay();
+  await delay();
+
+  Assert.ok(!dclFired, "DOMContentLoaded is blocked while parsing is blocked");
+
+  resolveBlockerPromise();
+  await dclPromise;
+
+  Assert.ok(dclFired, "DOMContentLoaded is fired once the blocker settles");
+
+  let page = await pagePromise;
+  await page.close();
+});

dom/prototype/PrototypeDocumentContentSink.cpp

@@ -694,7 +694,7 @@ nsresult PrototypeDocumentContentSink::DoneWalking() {
 
   doc->SetScrollToRef(mDocument->GetDocumentURI());
 
-  doc->EndLoad();
+  doc->EndLoad(/* aFireDOMContentLoadedSync = */ true);
 
   return NS_OK;
 }

dom/script/ScriptLoader.cpp

@@ -4638,9 +4638,11 @@ void ScriptLoader::ProcessPendingRequests(bool aAllowBypassingParserBlocking) {
 
   if (mDeferCheckpointReached && mDocument && !mParserBlockingRequest &&
       mNonAsyncExternalScriptInsertedRequests.isEmpty() &&
-      mXSLTRequests.isEmpty() && mDeferRequests.isEmpty() &&
-      MaybeRemovedDeferRequests()) {
-    return ProcessPendingRequests();
+      mXSLTRequests.isEmpty() && mDeferRequests.isEmpty()) {
+    const RefPtr<ScriptLoader> self = this;
+    if (self->MaybeRemovedDeferRequests()) {
+      return self->ProcessPendingRequests();
+    }
   }
 
   if (mDeferCheckpointReached && mDocument && !mParserBlockingRequest &&
@@ -5669,7 +5671,7 @@ void ScriptLoader::MaybeMoveToLoadedList(ScriptLoadRequest* aRequest) {
 bool ScriptLoader::MaybeRemovedDeferRequests() {
   if (mDeferRequests.isEmpty() && mDocument && mBlockingDOMContentLoaded) {
     mBlockingDOMContentLoaded = false;
-    mDocument->UnblockDOMContentLoaded();
+    mDocument->UnblockDOMContentLoaded(/* aFireSync = */ true);
     return true;
   }
   return false;

dom/script/ScriptLoader.h

@@ -862,7 +862,7 @@ class ScriptLoader final : public JS::loader::ScriptLoaderInterface {
 
   void AddDeferRequest(ScriptLoadRequest* aRequest);
   void AddAsyncRequest(ScriptLoadRequest* aRequest);
-  bool MaybeRemovedDeferRequests();
+  MOZ_CAN_RUN_SCRIPT bool MaybeRemovedDeferRequests();
 
   bool ShouldApplyDelazifyStrategy(ScriptLoadRequest* aRequest);
   void ApplyDelazifyStrategy(JS::CompileOptions* aOptions);

dom/xml/XMLDocument.cpp

@@ -298,11 +298,11 @@ nsresult XMLDocument::StartDocumentLoad(
   return NS_OK;
 }
 
-void XMLDocument::EndLoad() {
+void XMLDocument::EndLoad(bool aFireDOMContentLoadedSync) {
   mChannelIsPending = false;
 
   mSynchronousDOMContentLoaded = mLoadedAsData;
-  Document::EndLoad();
+  Document::EndLoad(aFireDOMContentLoadedSync);
   if (mSynchronousDOMContentLoaded) {
     mSynchronousDOMContentLoaded = false;
     Document::SetReadyStateInternal(Document::READYSTATE_COMPLETE);

dom/xml/XMLDocument.h

@@ -40,7 +40,8 @@ class XMLDocument : public Document {
                                      bool aReset = true) override;
 
   // TODO: Convert this to MOZ_CAN_RUN_SCRIPT (bug 1415230, bug 1535398)
-  MOZ_CAN_RUN_SCRIPT_BOUNDARY virtual void EndLoad() override;
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY virtual void EndLoad(
+      bool aFireDOMContentLoadedSync) override;
 
   virtual nsresult Init(nsIPrincipal* aPrincipal,
                         nsIPrincipal* aPartitionedPrincipal) override;

dom/xml/nsXMLContentSink.cpp

@@ -305,6 +305,7 @@ nsXMLContentSink::DidBuildModel(bool aTerminated) {
     mDocument->RemoveObserver(this);
     mIsDocumentObserver = false;
 
+    const RefPtr<nsXMLContentSink> kungFuDeathGrip(this);
     RefPtr<Document> doc = mDocument;
     if (!mDeferredLayoutStart && doc->IsBeingUsedAsImage()) {
       // Eagerly layout image documents, so that layout-triggered loads have a
@@ -312,7 +313,7 @@ nsXMLContentSink::DidBuildModel(bool aTerminated) {
       doc->FlushPendingNotifications(FlushType::Layout);
     }
 
-    doc->EndLoad();
+    doc->EndLoad(/* aFireDOMContentLoadedSync = */ !aTerminated);
 
     DropParserAndPerfHint();
   }
@@ -402,7 +403,7 @@ nsresult nsXMLContentSink::OnTransformDone(Document* aSourceDocument,
     ScrollToRef();
   }
 
-  originalDocument->EndLoad();
+  originalDocument->EndLoad(/* aFireDOMContentLoadedSync = */ true);
   if (blockingOnload) {
     // This UnblockOnload call corresponds to the BlockOnload call in
     // nsContentSink::WillBuildModelImpl.

modules/libpref/init/StaticPrefList.yaml

@@ -2817,6 +2817,12 @@
   value: true
   mirror: always
 
+# Fire DOMContentLoaded at the end of parsing, instead of from a task.
+- name: dom.document.domcontentloaded.synchronous.enabled
+  type: bool
+  value: true
+  mirror: always
+
 # Enable/disable Gecko specific edit commands
 - name: dom.document.edit_command.contentReadOnly.enabled
   type: bool

parser/html/nsHtml5TreeOpExecutor.cpp

@@ -258,7 +258,8 @@ nsHtml5TreeOpExecutor::DidBuildModel(bool aTerminated) {
   // This comes from nsXMLContentSink and the old (now removed)
   // nsHTMLContentSink. If this parser has been marked as broken, treat the end
   // of parse as forced termination.
-  DidBuildModelImpl(aTerminated || NS_FAILED(IsBroken()));
+  const bool terminated = aTerminated || NS_FAILED(IsBroken());
+  DidBuildModelImpl(terminated);
 
   bool destroying = true;
   if (mDocShell) {
@@ -290,8 +291,11 @@ nsHtml5TreeOpExecutor::DidBuildModel(bool aTerminated) {
 
   // We may not have called BeginLoad() if loading is terminated before
   // OnStartRequest call.
+  // EndLoad below can run script which drops the parser.
+  const RefPtr<nsHtml5Parser> parser = GetParser();
+
   if (mStarted) {
-    mDocument->EndLoad();
+    mDocument->EndLoad(/* aFireDOMContentLoadedSync = */ !terminated);
 
     // Log outcome only for top-level content navigations in order to
     // avoid noise from ad iframes.
@@ -311,7 +315,7 @@ nsHtml5TreeOpExecutor::DidBuildModel(bool aTerminated) {
     // error pages.
     bool httpOk = false;
     nsCOMPtr<nsIChannel> channel;
-    nsresult rv = GetParser()->GetChannel(getter_AddRefs(channel));
+    nsresult rv = parser->GetChannel(getter_AddRefs(channel));
     if (NS_SUCCEEDED(rv) && channel) {
       nsCOMPtr<nsIHttpChannel> httpChannel = do_QueryInterface(channel);
       if (httpChannel) {
@@ -422,7 +426,7 @@ nsHtml5TreeOpExecutor::DidBuildModel(bool aTerminated) {
   // before this executor's nsHtml5Parser has been made unreachable from its
   // nsHTMLDocument. (mDocument->EndLoad() above drops the parser from the
   // document.)
-  GetParser()->DropStreamParser();
+  parser->DropStreamParser();
   DropParserAndPerfHint();
 #ifdef GATHER_DOCWRITE_STATISTICS
   printf("UNSAFE SCRIPTS: %d\n", sUnsafeDocWrites);

testing/web-platform/meta/html/syntax/parsing/DOMContentLoaded-defer-task-order.tentative.html.ini

@@ -0,0 +1,2 @@
+[DOMContentLoaded-defer-task-order.tentative.html]
+  prefs: [dom.document.domcontentloaded.synchronous.enabled:true]

testing/web-platform/meta/html/syntax/parsing/DOMContentLoaded-defer.html.ini

@@ -1,3 +1,6 @@
 [DOMContentLoaded-defer.html]
-  disabled: https://bugzilla.mozilla.org/show_bug.cgi?id=1242128
-
+  prefs: [dom.document.domcontentloaded.synchronous.enabled:true]
+  # This test asserts an order which is not specified, see
+  # https://github.com/web-platform-tests/wpt/issues/4267
+  [The end: DOMContentLoaded and defer scripts]
+    expected: FAIL

testing/web-platform/tests/html/syntax/parsing/DOMContentLoaded-defer-task-order.tentative.html

@@ -0,0 +1,28 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>The end: DOMContentLoaded and a task queued from a defer script</title>
+<link rel=help href="https://html.spec.whatwg.org/multipage/#the-end">
+<link rel=help href="https://github.com/web-platform-tests/wpt/issues/4267">
+<!--
+  Tentative: the order between DOMContentLoaded, which the specification queues
+  on the DOM manipulation task source, and a timer task is not specified.
+  DOMContentLoaded-defer.html asserts the opposite order.
+-->
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="log"></div>
+<script>
+var order = [];
+document.addEventListener("DOMContentLoaded", () => order.push("DOMContentLoaded"));
+
+var t = async_test(
+    "DOMContentLoaded fires before a task queued from a defer script");
+
+function check() {
+  t.step(() => {
+    assert_array_equals(order, ["defer", "DOMContentLoaded", "timeout"]);
+  });
+  t.done();
+}
+</script>
+<script defer src="data:text/javascript,order.push('defer');t.step_timeout(()=>{order.push('timeout');check()},0)"></script>

testing/web-platform/tests/svg/linking/scripted/a.rel-noreferrer-policy.html

@@ -4,19 +4,17 @@
 <link rel="author" title="Divyansh Mangal" href="mailto:dmangal@microsoft.com">
 <link rel="help" href="https://w3c.github.io/svgwg/svg2-draft/linking.html#InterfaceSVGAElement">
 <title> Rel attribute with noreferrer value </title>
-<svg>
-  <a id="test" href="resources/a.rel-noreferrer-policy-target.html" rel="noreferrer"></a>
-  <script>
-    var anchorElement = document.getElementById('test');
+<body>
+<script>
+// The link is in an iframe, because a test must not navigate its own top level
+// browsing context.
+async_test(t => {
+  window.addEventListener("message", t.step_func_done(event => {
+    assert_equals(event.data, "");
+  }));
 
-    // Simulate a click event
-    var event = new MouseEvent('click', {
-      view: window,
-      bubbles: true,
-      cancelable: true
-    });
-
-    // Dispatch the event to the anchor element
-    anchorElement.dispatchEvent(event);
-  </script>
-</svg>
+  const iframe = document.createElement("iframe");
+  iframe.src = "resources/a.rel-noreferrer-policy-frame.html";
+  document.body.appendChild(iframe);
+}, "No Referrer policy attribute on svg anchor element is applied");
+</script>

testing/web-platform/tests/svg/linking/scripted/resources/a.rel-noreferrer-policy-frame.html

@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<svg>
+  <a id="test" href="a.rel-noreferrer-policy-target.html" rel="noreferrer"></a>
+  <script>
+    document.getElementById("test").dispatchEvent(new MouseEvent("click", {
+      view: window,
+      bubbles: true,
+      cancelable: true
+    }));
+  </script>
+</svg>

testing/web-platform/tests/svg/linking/scripted/resources/a.rel-noreferrer-policy-target.html

@@ -1,10 +1,5 @@
 <!DOCTYPE html>
-<script src="/resources/testharness.js"></script>
-<script src="/resources/testharnessreport.js"></script>
 <link rel="author" title="Divyansh Mangal" href="mailto:dmangal@microsoft.com">
-<link rel="help" href="https://w3c.github.io/svgwg/svg2-draft/linking.html#InterfaceSVGAElement">
 <script>
-  test(function () {
-    assert_equals("", document.referrer);
-  }, "No Referrer policy attribute on svg anchor element is applied");
+  parent.postMessage(document.referrer, "*");
 </script>

widget/tests/test_dnd_ignoreNuls.html

@@ -64,6 +64,8 @@
     }
 
     async function runTests() {
+      // The drag needs a focused window.
+      await SimpleTest.promiseFocus();
       await SpecialPowers.contentTransformsReceived(window);
       for (let currentTest of tests) {
         await test(currentTest.actual, currentTest.expected);