GWidget: Add some new child z-ordering facilities.

- child_at(Point)
- move_to_front()
- move_to_back()
- is_frontmost()
- is_backmost()

This patch also makes it possible to receive the mouse event that triggers
a context menu before the context menu is shown. I'm not sure this is the
best design for context menus but it works for now.

LibCore/CObject.h

@@ -18,6 +18,7 @@ public:
     virtual void event(CEvent&);
 
     Vector<CObject*>& children() { return m_children; }
+    const Vector<CObject*>& children() const { return m_children; }
 
     CObject* parent() { return m_parent; }
     const CObject* parent() const { return m_parent; }

LibGUI/GWidget.cpp

@@ -178,6 +178,8 @@ void GWidget::handle_mousedown_event(GMouseEvent& event)
         set_focus(true);
     if (event.button() == GMouseButton::Right) {
         if (m_context_menu) {
+            if (m_context_menu_mode == ContextMenuMode::PassthroughMouseEvent)
+                mousedown_event(event);
             m_context_menu->popup(screen_relative_rect().location().translated(event.position()));
             return;
         }
@@ -294,19 +296,26 @@ Rect GWidget::screen_relative_rect() const
     return window_relative_rect().translated(window()->position());
 }
 
-GWidget::HitTestResult GWidget::hit_test(int x, int y)
+GWidget* GWidget::child_at(const Point& point)
 {
-    if (is_greedy_for_hits())
-        return { this, x, y };
     for (int i = children().size() - 1; i >= 0; --i) {
         if (!children()[i]->is_widget())
             continue;
         auto& child = *(GWidget*)children()[i];
         if (!child.is_visible())
             continue;
-        if (child.relative_rect().contains(x, y))
-            return child.hit_test(x - child.relative_rect().x(), y - child.relative_rect().y());
+        if (child.relative_rect().contains(point))
+            return &child;
     }
+    return nullptr;
+}
+
+GWidget::HitTestResult GWidget::hit_test(int x, int y)
+{
+    if (is_greedy_for_hits())
+        return { this, x, y };
+    if (auto* child = child_at({ x, y }))
+        return child->hit_test(x - child->x(), y - child->y());
     return { this, x, y };
 }
 
@@ -426,9 +435,50 @@ void GWidget::set_enabled(bool enabled)
     update();
 }
 
-void GWidget::set_context_menu(OwnPtr<GMenu>&& context_menu)
+void GWidget::set_context_menu(OwnPtr<GMenu>&& context_menu, ContextMenuMode mode)
 {
     // FIXME: Support switching context menus.
     ASSERT(!m_context_menu);
     m_context_menu = move(context_menu);
+    m_context_menu_mode = mode;
+}
+
+void GWidget::move_to_front()
+{
+    auto* parent = parent_widget();
+    if (!parent)
+        return;
+    if (parent->children().size() == 1)
+        return;
+    parent->children().remove_first_matching([this] (auto& entry) { return entry == this; });
+    parent->children().append(this);
+    parent_widget()->update();
+}
+
+void GWidget::move_to_back()
+{
+    auto* parent = parent_widget();
+    if (!parent)
+        return;
+    if (parent->children().size() == 1)
+        return;
+    parent->children().remove_first_matching([this] (auto& entry) { return entry == this; });
+    parent->children().prepend(this);
+    parent_widget()->update();
+}
+
+bool GWidget::is_frontmost() const
+{
+    auto* parent = parent_widget();
+    if (!parent)
+        return true;
+    return parent->children().last() == this;
+}
+
+bool GWidget::is_backmost() const
+{
+    auto* parent = parent_widget();
+    if (!parent)
+        return true;
+    return parent->children().first() == this;
 }

LibGUI/GWidget.h

@@ -42,8 +42,10 @@ public:
     bool is_enabled() const { return m_enabled; }
     void set_enabled(bool);
 
+    enum class ContextMenuMode { SwallowMouseEvent, PassthroughMouseEvent };
+
     const GMenu* context_menu() const { return m_context_menu.ptr(); }
-    void set_context_menu(OwnPtr<GMenu>&&);
+    void set_context_menu(OwnPtr<GMenu>&&, ContextMenuMode = ContextMenuMode::SwallowMouseEvent);
 
     virtual void event(CEvent&) override;
     virtual void paint_event(GPaintEvent&);
@@ -95,6 +97,8 @@ public:
     };
     HitTestResult hit_test(int x, int y);
 
+    GWidget* child_at(const Point&);
+
     virtual const char* class_name() const override { return "GWidget"; }
 
     void set_relative_rect(const Rect&);
@@ -165,6 +169,12 @@ public:
     bool is_greedy_for_hits() const { return m_greedy_for_hits; }
     void set_greedy_for_hits(bool b) { m_greedy_for_hits = b; }
 
+    void move_to_front();
+    void move_to_back();
+
+    bool is_frontmost() const;
+    bool is_backmost() const;
+
 private:
     virtual bool is_widget() const final { return true; }
 
@@ -196,4 +206,5 @@ private:
 
     CElapsedTimer m_click_clock;
     OwnPtr<GMenu> m_context_menu;
+    ContextMenuMode m_context_menu_mode { ContextMenuMode::SwallowMouseEvent };
 };