LibWeb: Reschedule repaint for navigables with ongoing painting

Fixes delayed repainting in the following case:
1. Style or layout invalidation triggers html event loop processing.
2. Event loop processing does nothing because there is no rendering
   opportunity.
3. Style or layout change won't be reflected until something else
   triggers event loop processing

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

@@ -196,6 +196,22 @@ void EventLoop::process()
         }
     };
 
+    // AD-HOC: Since event loop processing steps do not constantly running in parallel, and
+    //         something must trigger them, we need to manually schedule a repaint for all
+    //         navigables that do not have a rendering opportunity at this event loop iteration.
+    //         Otherwise their repaint will be delayed until something else will trigger event
+    //         loop processing.
+    for_each_fully_active_document_in_docs([&](DOM::Document& document) {
+        auto navigable = document.navigable();
+        if (navigable && navigable->has_a_rendering_opportunity())
+            return;
+        auto* browsing_context = document.browsing_context();
+        if (!browsing_context)
+            return;
+        auto& page = browsing_context->page();
+        page.client().schedule_repaint();
+    });
+
     // 2. Rendering opportunities: Remove from docs all Document objects whose node navigables do not have a rendering opportunity.
     docs.remove_all_matching([&](auto& document) {
         auto navigable = document->navigable();