WindowServer: Move child windows together with their parents

When moving a window, we will now move any child windows by the same
position delta as the parent. This makes ComboBox popup list windows
follow the window they were opened by, which looks nice. :^)

Servers/WindowServer/Window.cpp

@@ -142,6 +142,25 @@ void Window::set_rect(const Gfx::Rect& rect)
     m_frame.notify_window_rect_changed(old_rect, rect);
 }
 
+void Window::set_rect_without_repaint(const Gfx::Rect& rect)
+{
+    ASSERT(!rect.is_empty());
+    if (m_rect == rect)
+        return;
+    auto old_rect = m_rect;
+    m_rect = rect;
+
+    if (old_rect.size() == m_rect.size()) {
+        auto delta = m_rect.location() - old_rect.location();
+        for (auto& child_window : m_child_windows) {
+            if (child_window)
+                child_window->move_by(delta);
+        }
+    }
+
+    m_frame.notify_window_rect_changed(old_rect, rect);
+}
+
 void Window::handle_mouse_event(const MouseEvent& event)
 {
     set_automatic_cursor_tracking_enabled(event.buttons() != 0);

Servers/WindowServer/Window.h

@@ -142,15 +142,7 @@ public:
     Gfx::Rect rect() const { return m_rect; }
     void set_rect(const Gfx::Rect&);
     void set_rect(int x, int y, int width, int height) { set_rect({ x, y, width, height }); }
-    void set_rect_without_repaint(const Gfx::Rect& rect)
-    {
-        ASSERT(!rect.is_empty());
-        if (m_rect == rect)
-            return;
-        auto old_rect = m_rect;
-        m_rect = rect;
-        m_frame.notify_window_rect_changed(old_rect, rect);
-    }
+    void set_rect_without_repaint(const Gfx::Rect&);
 
     void set_taskbar_rect(const Gfx::Rect& rect) { m_taskbar_rect = rect; }
     const Gfx::Rect& taskbar_rect() const { return m_taskbar_rect; }
@@ -158,6 +150,8 @@ public:
     void move_to(const Gfx::Point& position) { set_rect({ position, size() }); }
     void move_to(int x, int y) { move_to({ x, y }); }
 
+    void move_by(const Gfx::Point& delta) { set_position_without_repaint(position().translated(delta)); }
+
     Gfx::Point position() const { return m_rect.location(); }
     void set_position(const Gfx::Point& position) { set_rect({ position.x(), position.y(), width(), height() }); }
     void set_position_without_repaint(const Gfx::Point& position) { set_rect_without_repaint({ position.x(), position.y(), width(), height() }); }