LibWeb: Don't queue rendering task for empty display lists

Author: https://github.com/kalenikaliaksandr
Commit: https://github.com/LadybirdBrowser/ladybird/commit/bc0ec84100c
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4199
Reviewed-by: https://github.com/gmta ✅

Libraries/LibWeb/HTML/RenderingThread.cpp

@@ -56,7 +56,7 @@ void RenderingThread::rendering_thread_loop()
     }
 }
 
-void RenderingThread::enqueue_rendering_task(RefPtr<Painting::DisplayList> display_list, NonnullRefPtr<Gfx::PaintingSurface> painting_surface, Function<void()>&& callback)
+void RenderingThread::enqueue_rendering_task(NonnullRefPtr<Painting::DisplayList> display_list, NonnullRefPtr<Gfx::PaintingSurface> painting_surface, Function<void()>&& callback)
 {
     Threading::MutexLocker const locker { m_rendering_task_mutex };
     m_rendering_tasks.enqueue(Task { move(display_list), move(painting_surface), move(callback) });

Libraries/LibWeb/HTML/RenderingThread.h

@@ -25,7 +25,7 @@ public:
 
     void start();
     void set_skia_player(OwnPtr<Painting::DisplayListPlayerSkia>&& player) { m_skia_player = move(player); }
-    void enqueue_rendering_task(RefPtr<Painting::DisplayList>, NonnullRefPtr<Gfx::PaintingSurface>, Function<void()>&& callback);
+    void enqueue_rendering_task(NonnullRefPtr<Painting::DisplayList>, NonnullRefPtr<Gfx::PaintingSurface>, Function<void()>&& callback);
 
 private:
     void rendering_thread_loop();
@@ -38,7 +38,7 @@ private:
     Atomic<bool> m_exit { false };
 
     struct Task {
-        RefPtr<Painting::DisplayList> display_list;
+        NonnullRefPtr<Painting::DisplayList> display_list;
         NonnullRefPtr<Gfx::PaintingSurface> painting_surface;
         Function<void()> callback;
     };

Libraries/LibWeb/HTML/TraversableNavigable.cpp

@@ -1454,7 +1454,7 @@ RefPtr<Painting::DisplayList> TraversableNavigable::record_display_list(DevicePi
     return document->record_display_list(paint_config);
 }
 
-void TraversableNavigable::start_display_list_rendering(RefPtr<Painting::DisplayList> display_list, NonnullRefPtr<Gfx::PaintingSurface> painting_surface, Function<void()>&& callback)
+void TraversableNavigable::start_display_list_rendering(NonnullRefPtr<Painting::DisplayList> display_list, NonnullRefPtr<Gfx::PaintingSurface> painting_surface, Function<void()>&& callback)
 {
     m_rendering_thread.enqueue_rendering_task(move(display_list), move(painting_surface), move(callback));
 }

Libraries/LibWeb/HTML/TraversableNavigable.h

@@ -98,7 +98,7 @@ public:
     [[nodiscard]] GC::Ptr<DOM::Node> currently_focused_area();
 
     RefPtr<Painting::DisplayList> record_display_list(DevicePixelRect const&, PaintOptions);
-    void start_display_list_rendering(RefPtr<Painting::DisplayList> display_list, NonnullRefPtr<Gfx::PaintingSurface> painting_surface, Function<void()>&& callback);
+    void start_display_list_rendering(NonnullRefPtr<Painting::DisplayList> display_list, NonnullRefPtr<Gfx::PaintingSurface> painting_surface, Function<void()>&& callback);
 
     enum class CheckIfUnloadingIsCanceledResult {
         CanceledByBeforeUnload,

Services/WebContent/PageClient.cpp

@@ -242,8 +242,12 @@ void PageClient::start_display_list_rendering(Web::DevicePixelRect const& conten
     paint_options.has_focus = m_has_focus;
     auto& traversable = *page().top_level_traversable();
     auto display_list = traversable.record_display_list(content_rect, paint_options);
+    if (!display_list) {
+        callback();
+        return;
+    }
     auto painting_surface = traversable.painting_surface_for_backing_store(target);
-    traversable.start_display_list_rendering(display_list, painting_surface, move(callback));
+    traversable.start_display_list_rendering(*display_list, painting_surface, move(callback));
 }
 
 Queue<Web::QueuedInputEvent>& PageClient::input_event_queue()