From 1165a94624db0e4313a958ebba8290f1478923ff Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Sun, 6 Feb 2022 18:45:29 +0100
Subject: [PATCH] LibWeb: Implement BrowsingContext::currently_focused_area()

This is "currently focused area of a top level browsing context"
from the HTML spec.
---
 .../Libraries/LibWeb/HTML/BrowsingContext.cpp | 30 +++++++++++++++++++
 .../Libraries/LibWeb/HTML/BrowsingContext.h   |  2 ++
 2 files changed, 32 insertions(+)

diff --git a/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp b/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp
index fb15aa9335..76a7ec5999 100644
--- a/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp
+++ b/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp
@@ -8,6 +8,7 @@
 #include <LibWeb/DOM/HTMLCollection.h>
 #include <LibWeb/DOM/Window.h>
 #include <LibWeb/HTML/BrowsingContext.h>
+#include <LibWeb/HTML/BrowsingContextContainer.h>
 #include <LibWeb/HTML/EventLoop/EventLoop.h>
 #include <LibWeb/HTML/HTMLAnchorElement.h>
 #include <LibWeb/Layout/BreakNode.h>
@@ -362,4 +363,33 @@ bool BrowsingContext::has_a_rendering_opportunity() const
     return true;
 }
 
+// https://html.spec.whatwg.org/multipage/interaction.html#currently-focused-area-of-a-top-level-browsing-context
+RefPtr<DOM::Node> BrowsingContext::currently_focused_area()
+{
+    // 1. If topLevelBC does not have system focus, then return null.
+    if (!is_focused_context())
+        return nullptr;
+
+    // 2. Let candidate be topLevelBC's active document.
+    auto* candidate = active_document();
+
+    // 3. While candidate's focused area is a browsing context container with a non-null nested browsing context:
+    //    set candidate to the active document of that browsing context container's nested browsing context.
+    while (candidate->focused_element()
+        && is<HTML::BrowsingContextContainer>(candidate->focused_element())
+        && static_cast<HTML::BrowsingContextContainer&>(*candidate->focused_element()).nested_browsing_context()) {
+        candidate = static_cast<HTML::BrowsingContextContainer&>(*candidate->focused_element()).nested_browsing_context()->active_document();
+    }
+
+    // 4. If candidate's focused area is non-null, set candidate to candidate's focused area.
+    if (candidate->focused_element()) {
+        // NOTE: We return right away here instead of assigning to candidate,
+        //       since that would require compromising type safety.
+        return candidate->focused_element();
+    }
+
+    // 5. Return candidate.
+    return candidate;
+}
+
 }
diff --git a/Userland/Libraries/LibWeb/HTML/BrowsingContext.h b/Userland/Libraries/LibWeb/HTML/BrowsingContext.h
index 559a59ffae..b9b71debd5 100644
--- a/Userland/Libraries/LibWeb/HTML/BrowsingContext.h
+++ b/Userland/Libraries/LibWeb/HTML/BrowsingContext.h
@@ -104,6 +104,8 @@ public:
 
     bool has_a_rendering_opportunity() const;
 
+    RefPtr<DOM::Node> currently_focused_area();
+
 private:
     explicit BrowsingContext(Page&, HTML::BrowsingContextContainer*);