modifiable mouse wheel scroll step

include/nana/c++defines.hpp

@@ -86,7 +86,7 @@
 	#define NANA_LINUX
 	#define NANA_X11
 #else
-#	static_assert(false, "Only Windows and Unix are supported now (Mac OS is experimental)");
+	static_assert(false, "Only Windows and Unix are supported now (Mac OS is experimental)");
 #endif
 
 //Define a symbol for POSIX operating system.

include/nana/gui/basis.hpp

@@ -284,6 +284,18 @@ that return a corresponding nana::appearance with predefined values.
 		virtual void visible(bool visibility) = 0;
 		virtual bool visible() const = 0;
 	};//end class caret_interface
+
+	namespace parameters
+	{
+		/// The system-wide parameters for mouse wheel
+		struct mouse_wheel
+		{
+			unsigned lines;			///< The number of lines to scroll when the vertical mouse wheel is moved.
+			unsigned characters;	///< The number of characters to scroll when the horizontal mouse wheel is moved.
+
+			mouse_wheel();
+		};
+	}
 }//end namespace nana
 
 #include <nana/pop_ignore_diagnostic>

include/nana/gui/widgets/listbox.hpp

@@ -575,8 +575,6 @@ namespace nana
 				color_proxy header_floated{ static_cast<color_rgb>(0xBABBBC)};
 				color_proxy item_selected{ static_cast<color_rgb>(0xD5EFFC) };
 
-                  /// \todo how to implement some geometrical parameters ??
-
 				/// The max column width which is generated by fit_content is allowed. It is ignored when it is 0, or a max value is passed to fit_content.
 				unsigned max_fit_content{ 0 };
 
@@ -591,8 +589,7 @@ namespace nana
 				unsigned header_splitter_area_before{ 2 }; ///< def=2. But 4 is better... IMO
 				unsigned header_splitter_area_after { 3 }; ///< def=3. But 4 is better... 
 
-				//void debug_print(const std::string &msg);
-
+				::nana::parameters::mouse_wheel mouse_wheel{}; ///< The number of lines/characters to scroll when vertical/horizontal mouse wheel is moved.
 			};
 		}
 	}//end namespace drawerbase

include/nana/gui/widgets/skeletons/text_editor_part.hpp

@@ -28,6 +28,8 @@ namespace nana
 			{
 				color_proxy selection{static_cast<color_rgb>(0x3399FF)};
 				color_proxy selection_text{colors::white};
+
+				parameters::mouse_wheel mouse_wheel;	///< The number of lines/characters to scroll when the vertical/horizontal mouse wheel is moved.
 			};
 
 			class text_editor_event_interface