LibWeb+WebContent: Change event loop to synchronously paint next frame

...instead of scheduling repaint timer in PageClient.

This change fixes flickering on Discord that happened because:
- Event loop schedules repainting by activating repaint timer
- `Document::tear_down_layout_tree()` destroys paintable tree
- Repaint timer invokes callback and renders an empty frame because
  paintable tree was destroyed

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

@@ -327,7 +327,10 @@ void EventLoop::process()
         if (navigable && navigable->needs_repaint()) {
             auto* browsing_context = document.browsing_context();
             auto& page = browsing_context->page();
-            page.client().schedule_repaint();
+            if (navigable->is_traversable()) {
+                VERIFY(page.client().is_ready_to_paint());
+                page.client().paint_next_frame();
+            }
         }
     });
 

Userland/Libraries/LibWeb/HTML/Navigable.cpp

@@ -2133,8 +2133,9 @@ void Navigable::paint(Painting::RecordingPainter& recording_painter, PaintConfig
     auto background_color = document->background_color();
 
     recording_painter.fill_rect(bitmap_rect, background_color);
-    if (!document->paintable())
-        return;
+    if (!document->paintable()) {
+        VERIFY_NOT_REACHED();
+    }
 
     Web::PaintContext context(recording_painter, page.palette(), page.client().device_pixels_per_css_pixel());
     context.set_device_viewport_rect(viewport_rect);

Userland/Libraries/LibWeb/Page/Page.h

@@ -247,6 +247,7 @@ public:
     virtual DevicePixelRect screen_rect() const = 0;
     virtual double device_pixels_per_css_pixel() const = 0;
     virtual CSS::PreferredColorScheme preferred_color_scheme() const = 0;
+    virtual void paint_next_frame() = 0;
     virtual void paint(DevicePixelRect const&, Gfx::Bitmap&, PaintOptions = {}) = 0;
     virtual void page_did_change_title(ByteString const&) { }
     virtual void page_did_change_url(URL::URL const&) { }

Userland/Libraries/LibWeb/SVG/SVGDecodedImageData.h

@@ -73,6 +73,7 @@ public:
     virtual double device_pixels_per_css_pixel() const override { return 1.0; }
     virtual CSS::PreferredColorScheme preferred_color_scheme() const override { return m_host_page->client().preferred_color_scheme(); }
     virtual void request_file(FileRequest) override { }
+    virtual void paint_next_frame() override { }
     virtual void paint(DevicePixelRect const&, Gfx::Bitmap&, Web::PaintOptions = {}) override { }
     virtual void schedule_repaint() override { }
     virtual bool is_ready_to_paint() const override { return true; }

Userland/Services/WebContent/PageClient.cpp

@@ -57,23 +57,6 @@ PageClient::PageClient(PageHost& owner, u64 id)
 {
     setup_palette();
 
-    m_repaint_timer = Web::Platform::Timer::create_single_shot(0, [this] {
-        if (!m_backing_stores.back_bitmap) {
-            return;
-        }
-
-        auto& back_bitmap = *m_backing_stores.back_bitmap;
-        auto viewport_rect = page().css_to_device_rect(page().top_level_traversable()->viewport_rect());
-        paint(viewport_rect, back_bitmap);
-
-        auto& backing_stores = m_backing_stores;
-        swap(backing_stores.front_bitmap, backing_stores.back_bitmap);
-        swap(backing_stores.front_bitmap_id, backing_stores.back_bitmap_id);
-
-        m_paint_state = PaintState::WaitingForClient;
-        client().async_did_paint(m_id, viewport_rect.to_type<int>(), backing_stores.front_bitmap_id);
-    });
-
 #ifdef HAS_ACCELERATED_GRAPHICS
     if (s_use_gpu_painter) {
         auto context = AccelGfx::Context::create();
@@ -92,9 +75,6 @@ void PageClient::schedule_repaint()
         m_paint_state = PaintState::PaintWhenReady;
         return;
     }
-
-    if (!m_repaint_timer->is_active())
-        m_repaint_timer->start();
 }
 
 bool PageClient::is_ready_to_paint() const
@@ -197,6 +177,24 @@ Web::Layout::Viewport* PageClient::layout_root()
     return document->layout_node();
 }
 
+void PageClient::paint_next_frame()
+{
+    if (!m_backing_stores.back_bitmap) {
+        return;
+    }
+
+    auto& back_bitmap = *m_backing_stores.back_bitmap;
+    auto viewport_rect = page().css_to_device_rect(page().top_level_traversable()->viewport_rect());
+    paint(viewport_rect, back_bitmap);
+
+    auto& backing_stores = m_backing_stores;
+    swap(backing_stores.front_bitmap, backing_stores.back_bitmap);
+    swap(backing_stores.front_bitmap_id, backing_stores.back_bitmap_id);
+
+    m_paint_state = PaintState::WaitingForClient;
+    client().async_did_paint(m_id, viewport_rect.to_type<int>(), backing_stores.front_bitmap_id);
+}
+
 void PageClient::paint(Web::DevicePixelRect const& content_rect, Gfx::Bitmap& target, Web::PaintOptions paint_options)
 {
     Web::Painting::CommandList painting_commands;

Userland/Services/WebContent/PageClient.h

@@ -39,6 +39,7 @@ public:
 
     ErrorOr<void> connect_to_webdriver(ByteString const& webdriver_ipc_path);
 
+    virtual void paint_next_frame() override;
     virtual void paint(Web::DevicePixelRect const& content_rect, Gfx::Bitmap&, Web::PaintOptions = {}) override;
 
     void set_palette_impl(Gfx::PaletteImpl&);
@@ -177,7 +178,6 @@ private:
     };
 
     PaintState m_paint_state { PaintState::Ready };
-    RefPtr<Web::Platform::Timer> m_repaint_timer;
 
     Web::CSS::PreferredColorScheme m_preferred_color_scheme { Web::CSS::PreferredColorScheme::Auto };
 

Userland/Services/WebWorker/PageHost.h

@@ -29,6 +29,7 @@ public:
     virtual Web::DevicePixelRect screen_rect() const override;
     virtual double device_pixels_per_css_pixel() const override;
     virtual Web::CSS::PreferredColorScheme preferred_color_scheme() const override;
+    virtual void paint_next_frame() override {};
     virtual void paint(Web::DevicePixelRect const&, Gfx::Bitmap&, Web::PaintOptions = {}) override;
     virtual void request_file(Web::FileRequest) override;
     virtual void schedule_repaint() override {};