Write an OnDrawToken event handler to perform specific drawing whenever the specified part of the syntax memo's text has been painted. TextPos specifies position in text where X element is zero-based index of the first character in the line's string and Y element is zero-based index of the line. To get zero-based index in the whole text use CaretPosToStrPos function. CharCount specifies number of characters that have been painted. Rect specifies where specified part of the text has been painted. 

Drawing to the specified Rect will not be erased by the following painting, so in this event handler you may perform any specific drawing without flicker effects.

TSyntaxMemo Properties