LibWeb: Broadcast the viewport rect to clients immediately after layout

This lets elements figure out if they're visible within the viewport or
not, so they take appropriate action.

Fixes the issues with animations not starting until the viewport was
resized or scrolled.

3 files changed, 11 insertions, 0 deletions

diff --git a/Userland/Libraries/LibWeb/DOM/Document.cpp b/Userland/Libraries/LibWeb/DOM/Document.cpp
index 286403d5d4..9ea97816b6 100644
--- a/Userland/Libraries/LibWeb/DOM/Document.cpp
+++ b/Userland/Libraries/LibWeb/DOM/Document.cpp
@@ -889,6 +889,9 @@ void Document::update_layout()
 
     layout_state.commit();
 
+    // Broadcast the current viewport rect to any new paintables, so they know whether they're visible or not.
+    browsing_context()->inform_all_viewport_clients_about_the_current_viewport_rect();
+
     browsing_context()->set_needs_display();
 
     if (browsing_context()->is_top_level() && browsing_context()->active_document() == this) {
diff --git a/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp b/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp
index 874fce80b6..d0f6122ce2 100644
--- a/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp
+++ b/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp
@@ -539,6 +539,12 @@ void BrowsingContext::set_active_document(JS::NonnullGCPtr<DOM::Document> docume
         previously_active_document->did_stop_being_active_document_in_browsing_context({});
 }
 
+void BrowsingContext::inform_all_viewport_clients_about_the_current_viewport_rect()
+{
+    for (auto* client : m_viewport_clients)
+        client->browsing_context_did_set_viewport_rect(viewport_rect());
+}
+
 void BrowsingContext::set_viewport_rect(CSSPixelRect const& rect)
 {
     bool did_change = false;
diff --git a/Userland/Libraries/LibWeb/HTML/BrowsingContext.h b/Userland/Libraries/LibWeb/HTML/BrowsingContext.h
index d043fa6982..162179015d 100644
--- a/Userland/Libraries/LibWeb/HTML/BrowsingContext.h
+++ b/Userland/Libraries/LibWeb/HTML/BrowsingContext.h
@@ -277,6 +277,8 @@ public:
     virtual String const& window_handle() const override { return m_window_handle; }
     virtual void set_window_handle(String handle) override { m_window_handle = move(handle); }
 
+    void inform_all_viewport_clients_about_the_current_viewport_rect();
+
 private:
     explicit BrowsingContext(Page&, HTML::NavigableContainer*);