LibCore+LibGUI: Make CObject child events synchronous.

...and then make GWidget layout invalidation lazy. This way we coalesce
multiple invalidations into a single relayout and we don't have to worry
about child widgets not being fully constructed.

LibCore/CObject.cpp

@@ -4,8 +4,9 @@
 #include <AK/Assertions.h>
 #include <stdio.h>
 
-CObject::CObject(CObject* parent)
+CObject::CObject(CObject* parent, bool is_widget)
     : m_parent(parent)
+    , m_widget(is_widget)
 {
     if (m_parent)
         m_parent->add_child(*this);
@@ -43,7 +44,7 @@ void CObject::event(CEvent& event)
 void CObject::add_child(CObject& object)
 {
     m_children.append(&object);
-    CEventLoop::current().post_event(*this, make<CChildEvent>(CEvent::ChildAdded, object));
+    event(*make<CChildEvent>(CEvent::ChildAdded, object));
 }
 
 void CObject::remove_child(CObject& object)
@@ -51,7 +52,7 @@ void CObject::remove_child(CObject& object)
     for (ssize_t i = 0; i < m_children.size(); ++i) {
         if (m_children[i] == &object) {
             m_children.remove(i);
-            CEventLoop::current().post_event(*this, make<CChildEvent>(CEvent::ChildRemoved, object));
+            event(*make<CChildEvent>(CEvent::ChildRemoved, object));
             return;
         }
     }

LibCore/CObject.h

@@ -10,7 +10,7 @@ class CTimerEvent;
 
 class CObject : public Weakable<CObject> {
 public:
-    CObject(CObject* parent = nullptr);
+    CObject(CObject* parent = nullptr, bool is_widget = false);
     virtual ~CObject();
 
     virtual const char* class_name() const { return "CObject"; }
@@ -36,7 +36,7 @@ public:
 
     void deferred_invoke(Function<void(CObject&)>);
 
-    virtual bool is_widget() const { return false; }
+    bool is_widget() const { return m_widget; }
     virtual bool is_window() const { return false; }
 
 protected:
@@ -46,5 +46,6 @@ protected:
 private:
     CObject* m_parent { nullptr };
     int m_timer_id { 0 };
+    bool m_widget { false };
     Vector<CObject*> m_children;
 };

LibGUI/GWidget.cpp

@@ -11,7 +11,7 @@
 #include <unistd.h>
 
 GWidget::GWidget(GWidget* parent)
-    : CObject(parent)
+    : CObject(parent, true)
 {
     set_font(nullptr);
     m_background_color = Color::LightGray;
@@ -143,7 +143,7 @@ void GWidget::do_layout()
 
 void GWidget::notify_layout_changed(Badge<GLayout>)
 {
-    do_layout();
+    invalidate_layout();
 }
 
 void GWidget::handle_resize_event(GResizeEvent& event)
@@ -395,13 +395,19 @@ void GWidget::set_size_policy(SizePolicy horizontal_policy, SizePolicy vertical_
 
 void GWidget::invalidate_layout()
 {
-    auto* w = window();
-    if (!w)
+    if (m_layout_dirty)
         return;
-    if (!w->main_widget())
-        return;
-    do_layout();
-    w->main_widget()->do_layout();
+    m_layout_dirty = true;
+    deferred_invoke([this] (auto&) {
+        m_layout_dirty = false;
+        auto* w = window();
+        if (!w)
+            return;
+        if (!w->main_widget())
+            return;
+        do_layout();
+        w->main_widget()->do_layout();
+    });
 }
 
 void GWidget::set_visible(bool visible)

LibGUI/GWidget.h

@@ -200,6 +200,7 @@ private:
     bool m_visible { true };
     bool m_greedy_for_hits { false };
     bool m_enabled { true };
+    bool m_layout_dirty { false };
 
     CElapsedTimer m_click_clock;
 };