Valid from List & Label 13The option LL_OPTION_INCREMENTAL_PREVIEW which was added with List & Label 13 also influences the behavior of LlPreviewDisplay(). Typically, LlPreviesDisplay() is called at the end of a print loop. However, as the preview has been visible during the print already (if LL_OPTION_INCREMENTAL_PREVIEW is set to its default "1"), LlPreviewDisplay() doesn't do anything in order to prevent the preview from being displayed twice. This behavior allows existing code to immediately profit from the new incremental preview feature without any further code changes.
If, however, you'd like to call LlPreviewDisplay independently from your print loop in order to display existing preview files, set LL_OPTION_INCREMENTAL_PREVIEW to "0" beforehand.