LibWeb: Add a flag to pause an HTML event loop's execution

This will be used to unblock the WebContent IPC event loop while waiting
for a dialog response.

Userland/Libraries/LibWeb/HTML/EventLoop/EventLoop.h

@@ -73,6 +73,10 @@ public:
 
     double compute_deadline() const;
 
+    // https://html.spec.whatwg.org/multipage/webappapis.html#pause
+    void set_execution_paused(bool execution_paused) { m_execution_paused = execution_paused; }
+    bool execution_paused() const { return m_execution_paused; }
+
 private:
     Type m_type { Type::Window };
 
@@ -104,6 +108,8 @@ private:
 
     // https://html.spec.whatwg.org/multipage/browsing-the-web.html#termination-nesting-level
     size_t m_termination_nesting_level { 0 };
+
+    bool m_execution_paused { false };
 };
 
 EventLoop& main_thread_event_loop();

Userland/Libraries/LibWeb/HTML/EventLoop/TaskQueue.cpp

@@ -24,6 +24,9 @@ void TaskQueue::add(NonnullOwnPtr<Task> task)
 
 OwnPtr<Task> TaskQueue::take_first_runnable()
 {
+    if (m_event_loop.execution_paused())
+        return nullptr;
+
     for (size_t i = 0; i < m_tasks.size(); ++i) {
         if (m_tasks[i]->is_runnable())
             return m_tasks.take(i);
@@ -33,6 +36,9 @@ OwnPtr<Task> TaskQueue::take_first_runnable()
 
 bool TaskQueue::has_runnable_tasks() const
 {
+    if (m_event_loop.execution_paused())
+        return false;
+
     for (auto& task : m_tasks) {
         if (task->is_runnable())
             return true;