Main Window - Tree | Indicate Expandable                    Ctrl+Shift+O

Folder Popup Menu - Options | Indicate Expandable

 

 

Checking this menu item causes ThumbsPlus to only display a '+' beside folder entries which contain sub-folders and may be expanded, or '-' in folder entires which are already expanded. Otherwise, '+' is displayed for all folder entries, until the are actually opened. Then the '+' is updated to reflect the true existence of sub-folders.