Gets the smallest segment that contains all segments in this selection.
            May return null if the selection is empty.
            
Assembly: ICSharpCode.AvalonEdit (Module: ICSharpCode.AvalonEdit.dll) Version: 5.0.0.4238
Assembly: ICSharpCode.AvalonEdit (Module: ICSharpCode.AvalonEdit.dll) Version: 5.0.0.4238