LibWeb: Make CSS layout lazier

Instead of doing layout synchronously whenever something changes,
we now use a basic event loop timer to defer and coalesce relayouts.

If you did something that requires a relayout of the page, make sure
to call Document::set_needs_layout() and it will get coalesced with all
the other layout updates.

There's lots of room for improvement here, but this already makes many
web pages significantly snappier. :^)

Also, note that this exposes a number of layout bugs where we have been
relying on multiple relayouts to calculate the correct dimensions for
things. Now that we only do a single layout in many cases, these kind of
problems are much more noticeable. That should also make them easier to
figure out and fix. :^)

Userland/DevTools/HackStudio/Debugger/EvaluateExpressionDialog.cpp

@@ -144,8 +144,6 @@ void EvaluateExpressionDialog::set_output(const StringView& html)
     paragraph->set_inner_html(html);
 
     m_output_container->append_child(paragraph);
-    m_output_container->document().invalidate_layout();
-    m_output_container->document().update_layout();
 }
 
 }

Userland/Libraries/LibWeb/DOM/CharacterData.cpp

@@ -24,8 +24,7 @@ void CharacterData::set_data(String data)
     if (m_data == data)
         return;
     m_data = move(data);
-    // FIXME: This is definitely too aggressive.
-    document().schedule_forced_layout();
+    set_needs_style_update(true);
 }
 
 }

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -80,7 +80,7 @@ Document::Document(const AK::URL& url)
         update_style();
     });
 
-    m_forced_layout_timer = Core::Timer::create_single_shot(0, [this] {
+    m_layout_update_timer = Core::Timer::create_single_shot(0, [this] {
         force_layout();
     });
 }
@@ -164,11 +164,11 @@ void Document::schedule_style_update()
     m_style_update_timer->start();
 }
 
-void Document::schedule_forced_layout()
+void Document::schedule_layout_update()
 {
-    if (m_forced_layout_timer->is_active())
+    if (m_layout_update_timer->is_active())
         return;
-    m_forced_layout_timer->start();
+    m_layout_update_timer->start();
 }
 
 bool Document::is_child_allowed(const Node& node) const
@@ -402,25 +402,31 @@ AK::URL Document::parse_url(String const& url) const
     return m_url.complete_url(url);
 }
 
-void Document::invalidate_layout()
+void Document::set_needs_layout()
 {
-    tear_down_layout_tree();
+    if (m_needs_layout)
+        return;
+    m_needs_layout = true;
+    schedule_layout_update();
 }
 
 void Document::force_layout()
 {
-    invalidate_layout();
+    tear_down_layout_tree();
     update_layout();
 }
 
 void Document::ensure_layout()
 {
-    if (!m_layout_root)
+    if (m_needs_layout || !m_layout_root)
         update_layout();
 }
 
 void Document::update_layout()
 {
+    if (!m_needs_layout && m_layout_root)
+        return;
+
     if (!browsing_context())
         return;
 
@@ -438,6 +444,8 @@ void Document::update_layout()
         if (auto* page = this->page())
             page->client().page_did_layout();
     }
+
+    m_needs_layout = false;
 }
 
 static void update_style_recursively(DOM::Node& node)
@@ -461,7 +469,7 @@ void Document::update_style()
     if (!browsing_context())
         return;
     update_style_recursively(*this);
-    update_layout();
+    set_needs_layout();
 }
 
 RefPtr<Layout::Node> Document::create_layout_node()

Userland/Libraries/LibWeb/DOM/Document.h

@@ -144,19 +144,20 @@ public:
     void set_visited_link_color(Color);
 
     void force_layout();
-    void invalidate_layout();
     void ensure_layout();
 
     void update_style();
     void update_layout();
 
+    void set_needs_layout();
+
     virtual bool is_child_allowed(const Node&) const override;
 
     const Layout::InitialContainingBlock* layout_node() const;
     Layout::InitialContainingBlock* layout_node();
 
     void schedule_style_update();
-    void schedule_forced_layout();
+    void schedule_layout_update();
 
     NonnullRefPtr<HTMLCollection> get_elements_by_name(String const&);
     NonnullRefPtr<HTMLCollection> get_elements_by_class_name(FlyString const&);
@@ -352,7 +353,7 @@ private:
     Optional<Color> m_visited_link_color;
 
     RefPtr<Core::Timer> m_style_update_timer;
-    RefPtr<Core::Timer> m_forced_layout_timer;
+    RefPtr<Core::Timer> m_layout_update_timer;
 
     String m_source;
 
@@ -399,6 +400,8 @@ private:
 
     // Used by evaluate_media_queries_and_report_changes().
     Vector<WeakPtr<CSS::MediaQueryList>> m_media_query_lists;
+
+    bool m_needs_layout { false };
 };
 
 }

Userland/Libraries/LibWeb/DOM/Element.cpp

@@ -224,7 +224,7 @@ void Element::recompute_style()
         return;
     layout_node()->apply_style(*new_specified_css_values);
     if (diff == StyleDifference::NeedsRelayout) {
-        document().schedule_forced_layout();
+        document().set_needs_layout();
         return;
     }
     if (diff == StyleDifference::NeedsRepaint) {
@@ -270,7 +270,6 @@ ExceptionOr<void> Element::set_inner_html(String const& markup)
         return result.exception();
 
     set_needs_style_update(true);
-    document().invalidate_layout();
     return {};
 }
 

Userland/Libraries/LibWeb/DOM/Node.cpp

@@ -142,7 +142,6 @@ void Node::set_text_content(String const& content)
     }
 
     set_needs_style_update(true);
-    document().invalidate_layout();
 }
 
 RefPtr<Layout::Node> Node::create_layout_node()

Userland/Libraries/LibWeb/DOM/ShadowRoot.cpp

@@ -49,7 +49,6 @@ ExceptionOr<void> ShadowRoot::set_inner_html(String const& markup)
         return result.exception();
 
     set_needs_style_update(true);
-    document().invalidate_layout();
     return {};
 }
 

Userland/Libraries/LibWeb/HTML/HTMLElement.cpp

@@ -97,7 +97,6 @@ void HTMLElement::set_inner_text(StringView text)
     append_child(document().create_text_node(text));
 
     set_needs_style_update(true);
-    document().invalidate_layout();
 }
 
 String HTMLElement::inner_text()

Userland/Libraries/LibWeb/HTML/HTMLImageElement.cpp

@@ -21,7 +21,7 @@ HTMLImageElement::HTMLImageElement(DOM::Document& document, QualifiedName qualif
     , m_image_loader(*this)
 {
     m_image_loader.on_load = [this] {
-        this->document().update_layout();
+        this->document().schedule_style_update();
         queue_an_element_task(HTML::Task::Source::DOMManipulation, [this] {
             dispatch_event(DOM::Event::create(EventNames::load));
         });
@@ -29,7 +29,7 @@ HTMLImageElement::HTMLImageElement(DOM::Document& document, QualifiedName qualif
 
     m_image_loader.on_fail = [this] {
         dbgln("HTMLImageElement: Resource did fail: {}", src());
-        this->document().update_layout();
+        this->document().schedule_style_update();
         queue_an_element_task(HTML::Task::Source::DOMManipulation, [this] {
             dispatch_event(DOM::Event::create(EventNames::error));
         });

Userland/Libraries/LibWeb/Page/BrowsingContext.cpp

@@ -82,6 +82,8 @@ void BrowsingContext::set_viewport_rect(Gfx::IntRect const& rect)
 
     if (m_size != rect.size()) {
         m_size = rect.size();
+        if (auto* document = active_document())
+            document->set_needs_layout();
         did_change = true;
     }
 
@@ -105,6 +107,9 @@ void BrowsingContext::set_size(Gfx::IntSize const& size)
         return;
     m_size = size;
 
+    if (auto* document = active_document())
+        document->set_needs_layout();
+
     for (auto* client : m_viewport_clients)
         client->browsing_context_did_set_viewport_rect(viewport_rect());