LibWeb: Do not run microtasks when the event loop is paused

For example, running `alert(1)` will pause the event loop, during which
time no JavaScript should execute. This patch extends this disruption to
microtasks. This avoids a crash inside the microtask executor, which
asserts the JS execution context stack is empty.

This makes us behave the same as Firefox in the following page:

    <script>
        queueMicrotask(() => {
            console.log("inside microtask");
        });

        alert("hi");
    </script>

Before the aforementioned assertion was added, we would execute that
microtask before showing the alert. Firefox does not do this, and now
we don't either.

Libraries/LibWeb/HTML/EventLoop/EventLoop.cpp

@@ -503,6 +503,9 @@ void perform_a_microtask_checkpoint()
 // https://html.spec.whatwg.org/#perform-a-microtask-checkpoint
 void EventLoop::perform_a_microtask_checkpoint()
 {
+    if (execution_paused())
+        return;
+
     // NOTE: This assertion is per requirement 9.5 of the ECMA-262 spec, see: https://tc39.es/ecma262/#sec-jobs
     // > At some future point in time, when there is no running context in the agent for which the job is scheduled and that agent's execution context stack is empty...
     VERIFY(vm().execution_context_stack().is_empty());
@@ -653,6 +656,8 @@ EventLoop::PauseHandle::~PauseHandle()
 // https://html.spec.whatwg.org/multipage/webappapis.html#pause
 EventLoop::PauseHandle EventLoop::pause()
 {
+    m_execution_paused = true;
+
     // 1. Let global be the current global object.
     auto& global = current_principal_global_object();
 
@@ -667,7 +672,6 @@ EventLoop::PauseHandle EventLoop::pause()
     //    not run further tasks, and any script in the currently running task must block. User agents should remain
     //    responsive to user input while paused, however, albeit in a reduced capacity since the event loop will not be
     //    doing anything.
-    m_execution_paused = true;
 
     return PauseHandle { *this, global, time_before_pause };
 }

Tests/LibWeb/Text/expected/alert.txt

@@ -0,0 +1 @@
+PASS (didn't crash)

Tests/LibWeb/Text/input/alert.html

@@ -0,0 +1,7 @@
+<script src="include.js"></script>
+<script>
+    test(() => {
+        alert("Well hello friends!");
+        println("PASS (didn't crash)");
+    });
+</script>

UI/Headless/Test.cpp

@@ -379,6 +379,30 @@ static void run_test(HeadlessWebView& view, Test& test, Application& app)
 
     view.on_text_test_finish = {};
 
+    promise->when_resolved([&view, &test, &app](auto) {
+        auto url = URL::create_with_file_scheme(MUST(FileSystem::real_path(test.input_path)));
+
+        switch (test.mode) {
+        case TestMode::Text:
+        case TestMode::Layout:
+            run_dump_test(view, test, url, app.per_test_timeout_in_seconds * 1000);
+            return;
+        case TestMode::Ref:
+            run_ref_test(view, test, url, app.per_test_timeout_in_seconds * 1000);
+            return;
+        case TestMode::Crash:
+            run_dump_test(view, test, url, app.per_test_timeout_in_seconds * 1000);
+            return;
+        }
+
+        VERIFY_NOT_REACHED();
+    });
+
+    view.load("about:blank"sv);
+}
+
+static void set_ui_callbacks_for_tests(HeadlessWebView& view)
+{
     view.on_request_file_picker = [&](auto const& accepted_file_types, auto allow_multiple_files) {
         // Create some dummy files for tests.
         Vector<Web::HTML::SelectedFile> selected_files;
@@ -420,26 +444,10 @@ static void run_test(HeadlessWebView& view, Test& test, Application& app)
         view.file_picker_closed(move(selected_files));
     };
 
-    promise->when_resolved([&view, &test, &app](auto) {
-        auto url = URL::create_with_file_scheme(MUST(FileSystem::real_path(test.input_path)));
-
-        switch (test.mode) {
-        case TestMode::Text:
-        case TestMode::Layout:
-            run_dump_test(view, test, url, app.per_test_timeout_in_seconds * 1000);
-            return;
-        case TestMode::Ref:
-            run_ref_test(view, test, url, app.per_test_timeout_in_seconds * 1000);
-            return;
-        case TestMode::Crash:
-            run_dump_test(view, test, url, app.per_test_timeout_in_seconds * 1000);
-            return;
-        }
-
-        VERIFY_NOT_REACHED();
-    });
-
-    view.load("about:blank"sv);
+    view.on_request_alert = [&](auto const&) {
+        // For tests, just close the alert right away to unblock JS execution.
+        view.alert_closed();
+    };
 }
 
 ErrorOr<void> run_tests(Core::AnonymousBuffer const& theme, Web::DevicePixelSize window_size)
@@ -516,6 +524,7 @@ ErrorOr<void> run_tests(Core::AnonymousBuffer const& theme, Web::DevicePixelSize
     Vector<TestCompletion> non_passing_tests;
 
     app.for_each_web_view([&](auto& view) {
+        set_ui_callbacks_for_tests(view);
         view.clear_content_filters();
 
         auto run_next_test = [&]() {