LibWeb: Add simple ad-hoc version of window.postMessage()

This allows us to use the wpt.live copy of the ACID3 test, which is kept
updated, unlike the acidtests.org version.

Userland/Libraries/LibWeb/Bindings/WindowObject.cpp

@@ -95,6 +95,8 @@ void WindowObject::initialize_global_object()
     define_native_function("matchMedia", match_media, 1, attr);
     define_native_function("getSelection", get_selection, 0, attr);
 
+    define_native_function("postMessage", post_message, 1, attr);
+
     // FIXME: These properties should be [Replaceable] according to the spec, but [Writable+Configurable] is the closest we have.
     define_native_accessor("scrollX", scroll_x_getter, {}, attr);
     define_native_accessor("pageXOffset", scroll_x_getter, {}, attr);
@@ -641,6 +643,14 @@ JS_DEFINE_NATIVE_FUNCTION(WindowObject::screen_y_getter)
     return JS::Value(impl->screen_y());
 }
 
+JS_DEFINE_NATIVE_FUNCTION(WindowObject::post_message)
+{
+    auto* impl = TRY(impl_from(vm, global_object));
+    auto target_origin = TRY(vm.argument(1).to_string(global_object));
+    impl->post_message(vm.argument(0), target_origin);
+    return JS::js_undefined();
+}
+
 JS_DEFINE_NATIVE_FUNCTION(WindowObject::local_storage_getter)
 {
     auto* impl = TRY(impl_from(vm, global_object));

Userland/Libraries/LibWeb/Bindings/WindowObject.h

@@ -94,6 +94,8 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(screen_left_getter);
     JS_DECLARE_NATIVE_FUNCTION(screen_top_getter);
 
+    JS_DECLARE_NATIVE_FUNCTION(post_message);
+
     JS_DECLARE_NATIVE_FUNCTION(local_storage_getter);
 
     JS_DECLARE_NATIVE_FUNCTION(alert);

Userland/Libraries/LibWeb/DOM/Window.cpp

@@ -19,6 +19,7 @@
 #include <LibWeb/DOM/Window.h>
 #include <LibWeb/HTML/BrowsingContext.h>
 #include <LibWeb/HTML/EventLoop/EventLoop.h>
+#include <LibWeb/HTML/MessageEvent.h>
 #include <LibWeb/HTML/PageTransitionEvent.h>
 #include <LibWeb/HTML/Scripting/ExceptionReporter.h>
 #include <LibWeb/HTML/Storage.h>
@@ -476,4 +477,18 @@ Window* Window::parent()
     return &current->active_document()->window();
 }
 
+// https://html.spec.whatwg.org/multipage/web-messaging.html#window-post-message-steps
+DOM::ExceptionOr<void> Window::post_message(JS::Value message, String const&)
+{
+    // FIXME: This is an ad-hoc hack implementation instead, since we don't currently
+    //        have serialization and deserialization of messages.
+    HTML::queue_global_task(HTML::Task::Source::PostedMessage, *wrapper(), [strong_this = NonnullRefPtr(*this), message]() mutable {
+        HTML::MessageEventInit event_init {};
+        event_init.data = message;
+        event_init.origin = "<origin>";
+        strong_this->dispatch_event(HTML::MessageEvent::create(HTML::EventNames::message, event_init));
+    });
+    return {};
+}
+
 }

Userland/Libraries/LibWeb/DOM/Window.h

@@ -102,6 +102,8 @@ public:
 
     Window* parent();
 
+    DOM::ExceptionOr<void> post_message(JS::Value, String const& target_origin);
+
 private:
     explicit Window(Document&);