LibWeb: Don't update rendering in BrowsingContexts without opportunity

This patch adds the "has a rendering opportunity" concept from the spec
to BrowsingContext and uses it to filter out contexts that are unable
to render right now when doing the event loop's rendering updates.

Note that we actually consider all contexts to have a rendering
opportunity at all times right now. Coming up with reasons to avoid
rendering is left as a FIXME. :^)

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

@@ -12,6 +12,7 @@
 #include <LibWeb/DOM/Window.h>
 #include <LibWeb/HTML/EventLoop/EventLoop.h>
 #include <LibWeb/HighResolutionTime/Performance.h>
+#include <LibWeb/Page/BrowsingContext.h>
 
 namespace Web::HTML {
 
@@ -139,7 +140,10 @@ void EventLoop::process()
         }
     };
 
-    // FIXME:     2. Rendering opportunities: Remove from docs all Document objects whose browsing context do not have a rendering opportunity.
+    // 2. Rendering opportunities: Remove from docs all Document objects whose browsing context do not have a rendering opportunity.
+    docs.remove_all_matching([&](auto& document) {
+        return document->browsing_context() && !document->browsing_context()->has_a_rendering_opportunity();
+    });
 
     // 3. If docs is not empty, then set hasARenderingOpportunity to true.
     if (!docs.is_empty())

Userland/Libraries/LibWeb/Page/BrowsingContext.cpp

@@ -348,4 +348,14 @@ DOM::Document const* BrowsingContext::container_document() const
     return nullptr;
 }
 
+// https://html.spec.whatwg.org/#rendering-opportunity
+bool BrowsingContext::has_a_rendering_opportunity() const
+{
+    // A browsing context has a rendering opportunity if the user agent is currently able to present the contents of the browsing context to the user,
+    // accounting for hardware refresh rate constraints and user agent throttling for performance reasons, but considering content presentable even if it's outside the viewport.
+
+    // FIXME: We should at the very least say `false` here if we're an inactive browser tab.
+    return true;
+}
+
 }

Userland/Libraries/LibWeb/Page/BrowsingContext.h

@@ -102,6 +102,8 @@ public:
     DOM::Document* container_document();
     DOM::Document const* container_document() const;
 
+    bool has_a_rendering_opportunity() const;
+
 private:
     explicit BrowsingContext(Page&, HTML::BrowsingContextContainer*);