(svn r20456) -Codechange: Add separate nested widget class for scrollbars.

src/widget_type.h

@@ -501,6 +501,22 @@ public:
 	void UpdateViewportCoordinates(Window *w);
 };
 
+/**
+ * Nested widget to display and control a scrollbar in a window.
+ * Also assign the scrollbar to other widgets using #SetScrollbar() to make the mousewheel work.
+ * @ingroup NestedWidgets
+ */
+class NWidgetScrollbar : public NWidgetCore {
+public:
+	NWidgetScrollbar(WidgetType tp, Colours colour, int index);
+
+	/* virtual */ void SetupSmallestSize(Window *w, bool init_array);
+	/* virtual */ void Draw(const Window *w);
+
+	const Scrollbar *GetScrollbar(const Window *w) const;
+	Scrollbar *GetScrollbar(Window *w) const;
+};
+
 /**
  * Leaf widget.
  * @ingroup NestedWidgets