LibWeb: Implement window.queueMicrotask(callback)

This API allows authors to schedule a serialized JS callback that will
get invoked at the next spec-allowed opportunity.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -642,6 +642,10 @@ JS::Interpreter& Document::interpreter()
             auto& vm = m_interpreter->vm();
             vm.run_queued_promise_jobs();
             vm.run_queued_finalization_registry_cleanup_jobs();
+
+            // FIXME: This isn't exactly the right place for this.
+            HTML::main_thread_event_loop().perform_a_microtask_checkpoint();
+
             // Note: This is not an exception check for the promise jobs, they will just leave any
             // exception that already exists intact and never throw a new one (without cleaning it
             // up, that is). Taking care of any previous unhandled exception just happens to be the

Userland/Libraries/LibWeb/DOM/Window.cpp

@@ -12,6 +12,7 @@
 #include <LibWeb/DOM/EventDispatcher.h>
 #include <LibWeb/DOM/Timer.h>
 #include <LibWeb/DOM/Window.h>
+#include <LibWeb/HTML/EventLoop/EventLoop.h>
 #include <LibWeb/HTML/PageTransitionEvent.h>
 #include <LibWeb/HighResolutionTime/Performance.h>
 #include <LibWeb/Layout/InitialContainingBlock.h>
@@ -295,4 +296,17 @@ void Window::fire_a_page_transition_event(FlyString event_name, bool persisted)
     dispatch_event(move(event));
 }
 
+// https://html.spec.whatwg.org/#dom-queuemicrotask
+void Window::queue_microtask(JS::FunctionObject& callback)
+{
+    // The queueMicrotask(callback) method must queue a microtask to invoke callback,
+    HTML::queue_a_microtask(associated_document(), [&callback, handle = JS::make_handle(&callback)]() {
+        auto& vm = callback.vm();
+        [[maybe_unused]] auto rc = vm.call(callback, JS::js_null());
+        // FIXME: ...and if callback throws an exception, report the exception.
+        if (vm.exception())
+            vm.clear_exception();
+    });
+}
+
 }

Userland/Libraries/LibWeb/DOM/Window.h

@@ -55,6 +55,8 @@ public:
     void clear_timeout(i32);
     void clear_interval(i32);
 
+    void queue_microtask(JS::FunctionObject&);
+
     int inner_width() const;
     int inner_height() const;