LibWeb: Schedule repainting from EventLoop::process()

In this change, updating layout and painting are moved to the EventLoop
processing steps. This modification allows the addition of resize
observation dispatching that needs to happen in event loop processing
steps and must occur in the following order relative to layout and
painting:

1. Update layout.
2. Gather and broadcast resize observations.
3. Paint.

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

@@ -15,6 +15,7 @@
 #include <LibWeb/HTML/Window.h>
 #include <LibWeb/HighResolutionTime/Performance.h>
 #include <LibWeb/HighResolutionTime/TimeOrigin.h>
+#include <LibWeb/Page/Page.h>
 #include <LibWeb/Platform/EventLoopPlugin.h>
 #include <LibWeb/Platform/Timer.h>
 
@@ -213,7 +214,12 @@ void EventLoop::process()
 
     // FIXME:     15. Invoke the mark paint timing algorithm for each Document object in docs.
 
-    // FIXME:     16. For each fully active Document in docs, update the rendering or user interface of that Document and its browsing context to reflect the current state.
+    // 16. For each fully active Document in docs, update the rendering or user interface of that Document and its browsing context to reflect the current state.
+    for_each_fully_active_document_in_docs([&](DOM::Document& document) {
+        auto* browsing_context = document.browsing_context();
+        auto& page = browsing_context->page();
+        page.client().schedule_repaint();
+    });
 
     // 13. If all of the following are true
     // - this is a window event loop

Userland/Libraries/LibWeb/Page/Page.h

@@ -302,6 +302,8 @@ public:
     virtual void inspector_did_request_dom_tree_context_menu([[maybe_unused]] i32 node_id, [[maybe_unused]] CSSPixelPoint position, [[maybe_unused]] String const& type, [[maybe_unused]] Optional<String> const& tag, [[maybe_unused]] Optional<String> const& attribute_name, [[maybe_unused]] Optional<String> const& attribute_value) { }
     virtual void inspector_did_execute_console_script([[maybe_unused]] String const& script) { }
 
+    virtual void schedule_repaint() = 0;
+
 protected:
     virtual ~PageClient() = default;
 };

Userland/Libraries/LibWeb/SVG/SVGDecodedImageData.cpp

@@ -48,6 +48,7 @@ public:
     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(DevicePixelRect const&, Gfx::Bitmap&, Web::PaintOptions = {}) override { }
+    virtual void schedule_repaint() override { }
 
 private:
     explicit SVGPageClient(Page& host_page)

Userland/Services/WebContent/PageClient.cpp

@@ -222,12 +222,12 @@ void PageClient::paint(Web::DevicePixelRect const& content_rect, Gfx::Bitmap& ta
 void PageClient::set_viewport_rect(Web::DevicePixelRect const& rect)
 {
     page().top_level_traversable()->set_viewport_rect(page().device_to_css_rect(rect));
-    schedule_repaint();
+    Web::HTML::main_thread_event_loop().schedule();
 }
 
 void PageClient::page_did_invalidate(Web::CSSPixelRect const&)
 {
-    schedule_repaint();
+    Web::HTML::main_thread_event_loop().schedule();
 }
 
 void PageClient::page_did_request_cursor_change(Gfx::StandardCursor cursor)

Userland/Services/WebContent/PageClient.h

@@ -28,7 +28,7 @@ public:
 
     static void set_use_gpu_painter();
 
-    void schedule_repaint();
+    virtual void schedule_repaint() override;
 
     virtual Web::Page& page() override { return *m_page; }
     virtual Web::Page const& page() const override { return *m_page; }

Userland/Services/WebWorker/PageHost.h

@@ -30,6 +30,7 @@ public:
     virtual Web::CSS::PreferredColorScheme preferred_color_scheme() const override;
     virtual void paint(Web::DevicePixelRect const&, Gfx::Bitmap&, Web::PaintOptions = {}) override;
     virtual void request_file(Web::FileRequest) override;
+    virtual void schedule_repaint() override {};
 
 private:
     explicit PageHost(ConnectionFromClient&);