diff --git a/Libraries/LibWeb/DOM/Document.cpp b/Libraries/LibWeb/DOM/Document.cpp
index 0b7482f51b..939cad4ec7 100644
--- a/Libraries/LibWeb/DOM/Document.cpp
+++ b/Libraries/LibWeb/DOM/Document.cpp
@@ -344,15 +344,11 @@ WebIDL::ExceptionOr<GC::Ref<Document>> Document::create_and_initialize(Type type
     // 6. If browsingContext's active document's is initial about:blank is true,
     //    and browsingContext's active document's origin is same origin-domain with navigationParams's origin,
     //    then set window to browsingContext's active window.
-    // FIXME: still_on_its_initial_about_blank_document() is not in the spec anymore.
-    //        However, replacing this with the spec-mandated is_initial_about_blank() results in the browsing context
-    //        holding an incorrect active document for the replace from initial about:blank to the real document.
-    //        See #22293 for more details.
-    if (false
-        && (browsing_context->active_document() && browsing_context->active_document()->origin().is_same_origin(navigation_params.origin))) {
+    VERIFY(browsing_context->active_document());
+    if (browsing_context->active_document()->is_initial_about_blank()
+        && browsing_context->active_document()->origin().is_same_origin_domain(navigation_params.origin)) {
         window = browsing_context->active_window();
     }
-
     // 7. Otherwise:
     else {
         // FIXME: 1. Let oacHeader be the result of getting a structured field value given `Origin-Agent-Cluster` and "item" from response's header list.
@@ -5631,6 +5627,12 @@ void Document::make_active()
 
     set_window(window);
 
+    // AD-HOC: Keep the browsing context's active document distinct from the Window's associated Document.
+    //         The associated Document can be updated during document creation, but the browsing context
+    //         should only expose the new Document once it is made active.
+    //         Spec issue: https://github.com/whatwg/html/issues/12415
+    m_browsing_context->set_active_document(*this);
+
     // 2. Set document's browsing context's WindowProxy's [[Window]] internal slot value to window.
     m_browsing_context->window_proxy()->set_window(window);
 
diff --git a/Libraries/LibWeb/HTML/BrowsingContext.cpp b/Libraries/LibWeb/HTML/BrowsingContext.cpp
index e3eefe4fe1..02ea5ddd06 100644
--- a/Libraries/LibWeb/HTML/BrowsingContext.cpp
+++ b/Libraries/LibWeb/HTML/BrowsingContext.cpp
@@ -302,6 +302,7 @@ void BrowsingContext::visit_edges(Cell::Visitor& visitor)
 
     visitor.visit(m_page);
     visitor.visit(m_window_proxy);
+    visitor.visit(m_active_document);
     visitor.visit(m_group);
     visitor.visit(m_opener_browsing_context);
 }
@@ -347,21 +348,24 @@ GC::Ptr<BrowsingContext> BrowsingContext::top_level_browsing_context() const
 // https://html.spec.whatwg.org/multipage/document-sequences.html#active-document
 DOM::Document const* BrowsingContext::active_document() const
 {
-    // A browsing context's active document is its active window's associated Document.
-    auto* window = active_window();
-    if (!window)
-        return nullptr;
-    return &window->associated_document();
+    // AD-HOC: The HTML Standard currently defines this as the active window's associated Document.
+    //         That changes too early when the initial about:blank Window is reused for its first
+    //         same-origin navigation, because create-and-initialize updates the associated Document
+    //         before the new Document is made active.
+    //         Spec issue: https://github.com/whatwg/html/issues/12415
+    return m_active_document;
 }
 
 // https://html.spec.whatwg.org/multipage/document-sequences.html#active-document
 DOM::Document* BrowsingContext::active_document()
 {
-    // A browsing context's active document is its active window's associated Document.
-    auto* window = active_window();
-    if (!window)
-        return nullptr;
-    return &window->associated_document();
+    // AD-HOC: See the const overload above.
+    return m_active_document;
+}
+
+void BrowsingContext::set_active_document(GC::Ptr<DOM::Document> document)
+{
+    m_active_document = document;
 }
 
 // https://html.spec.whatwg.org/multipage/browsers.html#active-window
diff --git a/Libraries/LibWeb/HTML/BrowsingContext.h b/Libraries/LibWeb/HTML/BrowsingContext.h
index 390f9b3447..5d39126272 100644
--- a/Libraries/LibWeb/HTML/BrowsingContext.h
+++ b/Libraries/LibWeb/HTML/BrowsingContext.h
@@ -42,6 +42,7 @@ public:
 
     DOM::Document const* active_document() const;
     DOM::Document* active_document();
+    void set_active_document(GC::Ptr<DOM::Document>);
 
     HTML::WindowProxy* window_proxy();
     HTML::WindowProxy const* window_proxy() const;
@@ -93,6 +94,8 @@ private:
     // https://html.spec.whatwg.org/multipage/document-sequences.html#browsing-context
     GC::Ptr<WindowProxy> m_window_proxy;
 
+    GC::Ptr<DOM::Document> m_active_document;
+
     // https://html.spec.whatwg.org/multipage/browsers.html#opener-browsing-context
     GC::Ptr<BrowsingContext> m_opener_browsing_context;
 
diff --git a/Libraries/LibWeb/HTML/TraversableNavigable.cpp b/Libraries/LibWeb/HTML/TraversableNavigable.cpp
index 5f8d145894..41ee39b607 100644
--- a/Libraries/LibWeb/HTML/TraversableNavigable.cpp
+++ b/Libraries/LibWeb/HTML/TraversableNavigable.cpp
@@ -657,7 +657,10 @@ void ApplyHistoryStepState::start()
             signal_progress();
             continue;
         }
-        queue_global_task(Task::Source::NavigationAndTraversal, *navigable->active_window(), GC::create_function(heap(), [this, navigable] {
+        // AD-HOC: Queue with navigable's active Document instead of using queue_global_task(active_window).
+        //         During initial about:blank Window reuse, active_window()->associated_document() can already be
+        //         the pending Document. This activation task must stay runnable against the current active Document.
+        queue_a_task(Task::Source::NavigationAndTraversal, nullptr, navigable->active_document(), GC::create_function(heap(), [this, navigable] {
             // NOTE: This check is not in the spec but we should not continue navigation if navigable has been destroyed.
             if (navigable->has_been_destroyed()) {
                 ++m_completed_change_jobs;
@@ -827,9 +830,12 @@ void ApplyHistoryStepState::start()
                         *potentially_target_specific_source_snapshot_params, target_snapshot_params,
                         user_involvement, {}, Navigable::NullOrError {},
                         ContentSecurityPolicy::Directives::Directive::NavigationType::Other, allow_POST,
-                        GC::create_function(this->heap(), [this, after_document_populated](GC::Ptr<PopulateSessionHistoryEntryDocumentOutput> output) {
+                        GC::create_function(this->heap(), [this, after_document_populated, navigable](GC::Ptr<PopulateSessionHistoryEntryDocumentOutput> output) {
                             VERIFY(m_traversable->active_window());
-                            queue_global_task(Task::Source::NavigationAndTraversal, *m_traversable->active_window(), GC::create_function(heap(), [after_document_populated, output]() {
+                            // AD-HOC: Queue with navigable's active Document instead of using queue_global_task(active_window).
+                            //         During initial about:blank Window reuse, active_window()->associated_document() can already be
+                            //         the pending Document. This continuation must stay runnable against the current active Document.
+                            queue_a_task(Task::Source::NavigationAndTraversal, nullptr, navigable->active_document(), GC::create_function(heap(), [after_document_populated, output]() {
                                 after_document_populated->function()(output);
                             }));
                         }));
diff --git a/Tests/LibWeb/TestConfig.ini b/Tests/LibWeb/TestConfig.ini
index e6c1065ebb..aad27eed7e 100644
--- a/Tests/LibWeb/TestConfig.ini
+++ b/Tests/LibWeb/TestConfig.ini
@@ -189,6 +189,7 @@ Text/input/XHR/XMLHttpRequest-network-error-message.html
 Text/input/XHR/XMLHttpRequest-override-mimetype-blob.html
 Text/input/XML/error-page.html
 Text/input/base/a-element-target.html
+Text/input/navigation/iframe-initial-load-chunked-body.html
 Text/input/navigation/iframe-navigate-javascript-url.html
 Text/input/navigation/iframe-referrer-policy.html
 Text/input/parse-document-from-string-in-fetch-callback.html
@@ -230,6 +231,7 @@ Text/input/wpt-import/html/the-xhtml-syntax/parsing-xhtml-documents/xhtml-mathml
 Text/input/wpt-import/html/the-xhtml-syntax/parsing-xhtml-documents/xhtml-mathml-dtd-entity-9.htm
 Text/input/wpt-import/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/aborted-parser.window.html
 Text/input/wpt-import/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/url-entry-document-sync-call.window.html
+Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/promise-job-entry-different-function-realm.html
 Text/input/wpt-import/navigation-api/navigation-methods/reload-state-and-info.html
 Text/input/wpt-import/navigation-api/navigation-methods/reload-state-undefined.html
 Text/input/wpt-import/page-visibility/test_child_document.html
diff --git a/Tests/LibWeb/Text/expected/navigation/iframe-initial-load-chunked-body.txt b/Tests/LibWeb/Text/expected/navigation/iframe-initial-load-chunked-body.txt
new file mode 100644
index 0000000000..1d9f092306
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/navigation/iframe-initial-load-chunked-body.txt
@@ -0,0 +1 @@
+chunked iframe loaded
diff --git a/Tests/LibWeb/Text/expected/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/promise-job-entry-different-function-realm.txt b/Tests/LibWeb/Text/expected/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/promise-job-entry-different-function-realm.txt
new file mode 100644
index 0000000000..9effb466ea
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/promise-job-entry-different-function-realm.txt
@@ -0,0 +1,10 @@
+Harness status: OK
+
+Found 5 tests
+
+5 Pass
+Pass	Fulfillment handler on fulfilled promise
+Pass	Rejection handler on rejected promise
+Pass	Fulfillment handler on pending-then-fulfilled promise
+Pass	Rejection handler on pending-then-rejected promise
+Pass	Thenable resolution
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/input/navigation/iframe-initial-load-chunked-body.html b/Tests/LibWeb/Text/input/navigation/iframe-initial-load-chunked-body.html
new file mode 100644
index 0000000000..b90b2575c2
--- /dev/null
+++ b/Tests/LibWeb/Text/input/navigation/iframe-initial-load-chunked-body.html
@@ -0,0 +1,34 @@
+<!DOCTYPE html>
+<script src="../include.js"></script>
+<script>
+    const childHTML = [
+        "<!DOCTYPE html>",
+        "<title>chunked iframe</title>",
+        "<body>",
+        Array.from({ length: 4096 }, (_, index) => `<span>${index}</span>`).join(""),
+        "<p id=\"result\">chunked iframe loaded</p>",
+        "</body>",
+    ].join("");
+
+    const xhr = new XMLHttpRequest();
+    xhr.open("POST", "/echo", false);
+    xhr.setRequestHeader("Content-Type", "application/json");
+    xhr.send(JSON.stringify({
+        method: "GET",
+        path: "/echo/iframe-initial-load-chunked-body-child",
+        status: 200,
+        headers: { "Content-Type": "text/html; charset=utf-8" },
+        body: childHTML,
+    }));
+</script>
+<iframe id="target" src="/echo/iframe-initial-load-chunked-body-child?chunks=1024,1024,1024,1024&chunk_delay_ms=1"></iframe>
+<script>
+    asyncTest(done => {
+        const iframe = document.getElementById("target");
+
+        window.addEventListener("load", () => {
+            println(iframe.contentDocument.getElementById("result").textContent);
+            done();
+        }, { once: true });
+    });
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/promise-job-entry-different-function-realm.html b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/promise-job-entry-different-function-realm.html
new file mode 100644
index 0000000000..218a2a047a
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/promise-job-entry-different-function-realm.html
@@ -0,0 +1,112 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Entry settings object for promise jobs when the function realm is different from the test realm</title>
+<script src="../../../../../resources/testharness.js"></script>
+<script src="../../../../../resources/testharnessreport.js"></script>
+<!-- https://github.com/whatwg/html/pull/5212 -->
+<!-- https://github.com/whatwg/html/issues/1426 -->
+
+<!-- This is what would normally be considered the entry page. However, we use functions from the
+     resources/function/function.html realm. So window.open() should resolve relative to that realm
+     inside promise jobs. -->
+
+<iframe src="resources/promise-job-entry-incumbent.html"></iframe>
+<iframe src="resources/function/function.html" id="function-frame"></iframe>
+
+<script>
+setup({ explicit_done: true });
+
+const relativeURL = "resources/window-to-open.html";
+const expectedURL = (new URL(relativeURL, document.querySelector("#function-frame").src)).href;
+
+const incumbentWindow = frames[0];
+const functionWindow = frames[1];
+const FunctionFromAnotherWindow = frames[1].Function;
+
+window.onload = () => {
+  async_test(t => {
+    const func = FunctionFromAnotherWindow(`
+      const [incumbentWindow, relativeURL, t, assert_equals, expectedURL] = arguments[0];
+
+      const w = incumbentWindow.runWindowOpenVeryIndirectly(relativeURL);
+      w.onload = t.step_func_done(() => {
+        t.add_cleanup(() => w.close());
+        assert_equals(w.location.href, expectedURL);
+      });
+    `);
+
+    Promise.resolve([incumbentWindow, relativeURL, t, assert_equals, expectedURL]).then(func);
+  }, "Fulfillment handler on fulfilled promise");
+
+  async_test(t => {
+    const func = FunctionFromAnotherWindow(`
+      const [incumbentWindow, relativeURL, t, assert_equals, expectedURL] = arguments[0];
+
+      const w = incumbentWindow.runWindowOpenVeryIndirectly(relativeURL);
+      w.onload = t.step_func_done(() => {
+        t.add_cleanup(() => w.close());
+        assert_equals(w.location.href, expectedURL);
+      });
+    `);
+
+    Promise.reject([incumbentWindow, relativeURL, t, assert_equals, expectedURL]).catch(func);
+  }, "Rejection handler on rejected promise");
+
+  async_test(t => {
+    let resolve;
+    const p = new Promise(r => { resolve = r; });
+
+    const func = FunctionFromAnotherWindow(`
+      const [incumbentWindow, relativeURL, t, assert_equals, expectedURL] = arguments[0];
+
+      const w = incumbentWindow.runWindowOpenVeryIndirectly(relativeURL);
+      w.onload = t.step_func_done(() => {
+        t.add_cleanup(() => w.close());
+        assert_equals(w.location.href, expectedURL);
+      });
+    `);
+
+    p.then(func);
+    t.step_timeout(() => resolve([incumbentWindow, relativeURL, t, assert_equals, expectedURL]), 0);
+  }, "Fulfillment handler on pending-then-fulfilled promise");
+
+  async_test(t => {
+    let reject;
+    const p = new Promise((_, r) => { reject = r; });
+
+    const func = FunctionFromAnotherWindow(`
+      const [incumbentWindow, relativeURL, t, assert_equals, expectedURL] = arguments[0];
+
+      const w = incumbentWindow.runWindowOpenVeryIndirectly(relativeURL);
+      w.onload = t.step_func_done(() => {
+        t.add_cleanup(() => w.close());
+        assert_equals(w.location.href, expectedURL);
+      });
+    `);
+
+    p.catch(func);
+    t.step_timeout(() => reject([incumbentWindow, relativeURL, t, assert_equals, expectedURL]), 0);
+  }, "Rejection handler on pending-then-rejected promise");
+
+  async_test(t => {
+    t.add_cleanup(() => { delete frames[1].args; });
+    frames[1].args = [incumbentWindow, relativeURL, t, assert_equals, expectedURL];
+
+    const func = FunctionFromAnotherWindow(`
+      const [incumbentWindow, relativeURL, t, assert_equals, expectedURL] = window.args;
+
+      const w = incumbentWindow.runWindowOpenVeryIndirectly(relativeURL);
+      w.onload = t.step_func_done(() => {
+        t.add_cleanup(() => w.close());
+        assert_equals(w.location.href, expectedURL);
+      });
+    `);
+
+    const thenable = { then: func };
+
+    Promise.resolve(thenable);
+  }, "Thenable resolution");
+
+  done();
+};
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/current/current.html b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/current/current.html
new file mode 100644
index 0000000000..2c690a79d6
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/current/current.html
@@ -0,0 +1,3 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Current page used as a test helper</title>
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/current/resources/window-to-open.html b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/current/resources/window-to-open.html
new file mode 100644
index 0000000000..1bc4cca9a3
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/current/resources/window-to-open.html
@@ -0,0 +1,3 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>If the current settings object is used this page will be opened</title>
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/function/function.html b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/function/function.html
new file mode 100644
index 0000000000..15841d387d
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/function/function.html
@@ -0,0 +1,3 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Realm for a "then" function used as a test helper</title>
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/function/resources/window-to-open.html b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/function/resources/window-to-open.html
new file mode 100644
index 0000000000..3928c1f8aa
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/function/resources/window-to-open.html
@@ -0,0 +1,3 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>If the function's settings object is used this page will be opened</title>
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/promise-job-entry-incumbent.html b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/promise-job-entry-incumbent.html
new file mode 100644
index 0000000000..3740c1467d
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/promise-job-entry-incumbent.html
@@ -0,0 +1,15 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Incumbent page used as a test helper</title>
+
+<iframe src="relevant/relevant.html" id="r"></iframe>
+<iframe src="current/current.html" id="c"></iframe>
+
+<script>
+  const relevant = document.querySelector("#r");
+  const current = document.querySelector("#c");
+
+  window.runWindowOpenVeryIndirectly = (...args) => {
+    return current.contentWindow.open.call(relevant.contentWindow, ...args);
+  };
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/relevant/relevant.html b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/relevant/relevant.html
new file mode 100644
index 0000000000..f5965f2231
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/relevant/relevant.html
@@ -0,0 +1,14 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Relevant page used as a test helper</title>
+
+<script>
+// promise-job-incumbent will end up posting a message to here. We need to signal back the "source".
+
+window.onmessage = e => {
+  const testId = e.data;
+  const sourceURL = e.source.document.URL;
+
+  window.dispatchEvent(new CustomEvent("messagereceived", { detail: [testId, sourceURL] }));
+};
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/relevant/resources/window-to-open.html b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/relevant/resources/window-to-open.html
new file mode 100644
index 0000000000..4138b5a084
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/relevant/resources/window-to-open.html
@@ -0,0 +1,3 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>If the relevant settings object is used this page will be opened</title>
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/window-to-open.html b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/window-to-open.html
new file mode 100644
index 0000000000..ce357937f5
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/webappapis/scripting/processing-model-2/integration-with-the-javascript-job-queue/resources/window-to-open.html
@@ -0,0 +1,3 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>If the entry settings object is used this page will be opened</title>