LibWeb: Flesh out "document visibility" state a bit more

We can now "update the visibility state", which also causes
`visibilitychange` events to fire on the document.

This still needs GUI integration work at the BrowsingContext level.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -1578,13 +1578,31 @@ Bindings::LocationObject* Document::location()
 // https://html.spec.whatwg.org/multipage/interaction.html#dom-document-hidden
 bool Document::hidden() const
 {
-    return false;
+    return visibility_state() == "hidden";
 }
 
 // https://html.spec.whatwg.org/multipage/interaction.html#dom-document-visibilitystate
 String Document::visibility_state() const
 {
-    return hidden() ? "hidden" : "visible";
+    return m_visibility_state;
+}
+
+// https://html.spec.whatwg.org/multipage/interaction.html#update-the-visibility-state
+void Document::update_the_visibility_state(String visibility_state)
+{
+    // 1. If document's visibility state equals visibilityState, then return.
+    if (m_visibility_state == visibility_state)
+        return;
+
+    // 2. Set document's visibility state to visibilityState.
+    m_visibility_state = visibility_state;
+
+    // FIXME: 3. Run any page visibility change steps which may be defined in other specifications, with visibility state and document.
+
+    // 4. Fire an event named visibilitychange at document, with its bubbles attribute initialized to true.
+    auto event = DOM::Event::create(window(), HTML::EventNames::visibilitychange);
+    event->set_bubbles(true);
+    dispatch_event(event);
 }
 
 // https://drafts.csswg.org/cssom-view/#run-the-resize-steps

Userland/Libraries/LibWeb/DOM/Document.h

@@ -317,6 +317,9 @@ public:
     bool hidden() const;
     String visibility_state() const;
 
+    // https://html.spec.whatwg.org/multipage/interaction.html#update-the-visibility-state
+    void update_the_visibility_state(String visibility_state);
+
     void run_the_resize_steps();
     void run_the_scroll_steps();
 
@@ -502,6 +505,9 @@ private:
 
     // https://html.spec.whatwg.org/#completely-loaded-time
     Optional<AK::Time> m_completely_loaded_time;
+
+    // https://html.spec.whatwg.org/multipage/interaction.html#visibility-state
+    String m_visibility_state { "hidden" };
 };
 
 }

Userland/Libraries/LibWeb/HTML/EventNames.h

@@ -57,7 +57,8 @@ namespace Web::HTML::EventNames {
     __ENUMERATE_HTML_EVENT(submit)                  \
     __ENUMERATE_HTML_EVENT(toggle)                  \
     __ENUMERATE_HTML_EVENT(unhandledrejection)      \
-    __ENUMERATE_HTML_EVENT(unload)
+    __ENUMERATE_HTML_EVENT(unload)                  \
+    __ENUMERATE_HTML_EVENT(visibilitychange)
 
 #define __ENUMERATE_HTML_EVENT(name) extern FlyString name;
 ENUMERATE_HTML_EVENTS