BCGControlBar Pro for MFC
void CBCGPEditCtrl::SetOutlineMargin ( BOOL  bShow = TRUE,
int  nMarginWidth = 20 

Sets outline margin of the editor.

Used to set outline margin of the editor. If it is enabled, the edit control has a special area at the left side (outline margin) used to display expansion marks for each collapsible node.

bShowSpecifies whether the outline margin is visible or not.
nMarginWidthSpecifies the width of the outline margin in pixels