LibGUI: Implement new layout system for Scrollbar

Userland/Libraries/LibGUI/Scrollbar.cpp

@@ -48,11 +48,7 @@ Scrollbar::Scrollbar(Orientation orientation)
 {
     m_automatic_scrolling_timer = add<Core::Timer>();
 
-    if (orientation == Orientation::Vertical) {
+    set_preferred_size({ SpecialDimension::Fit });
-        set_fixed_width(16);
-    } else {
-        set_fixed_height(16);
-    }
 
     m_automatic_scrolling_timer->set_interval(100);
     m_automatic_scrolling_timer->on_timeout = [this] {
@@ -421,4 +417,20 @@ void Scrollbar::update_animated_scroll()
     AbstractSlider::set_value(new_value);
 }
 
+Optional<UISize> Scrollbar::calculated_min_size() const
+{
+    if (orientation() == Gfx::Orientation::Vertical)
+        return { { default_button_size(), 2 * default_button_size() } };
+    else
+        return { { 2 * default_button_size(), default_button_size() } };
+}
+
+Optional<UISize> Scrollbar::calculated_preferred_size() const
+{
+    if (orientation() == Gfx::Orientation::Vertical)
+        return { { SpecialDimension::Shrink, SpecialDimension::Grow } };
+    else
+        return { { SpecialDimension::Grow, SpecialDimension::Shrink } };
+}
+
 }

Userland/Libraries/LibGUI/Scrollbar.h

@@ -40,6 +40,9 @@ public:
     virtual void increase_slider_by_steps(int steps) override { set_target_value(m_target_value + step() * steps); }
     virtual void decrease_slider_by_steps(int steps) override { set_target_value(m_target_value - step() * steps); }
 
+    virtual Optional<UISize> calculated_min_size() const override;
+    virtual Optional<UISize> calculated_preferred_size() const override;
+
     enum Component {
         None,
         DecrementButton,