Fields
- direction: := Scrollable_Region_Flags.VERTICAL
- horizontal_margin_size: := 0.005
- vertical_margin_size: := 0.005
- margin_flags: := Margin.Horizontal | .Vertical
- scrollbar_size: := 0.03
- minimum_nib_size: := 1.0
- nib_margin: := 0.10
- scrollbar_nib_theme: Button_Theme
- region_background: Background_Theme
- scrollbar_background: Background_Theme
- mouse_wheel_increment: := 0.0004