It seems to me that you can achieve this without getting the current position of text area’s cursor.
Configure OnChange post value on text area, so when you move out of text area you’ll always have the updated value and onChange of drop down or whatever field, post it’s value too and append it to textarea content using a DT probably and reload the Textarea section.
Have you already tried this? Any challenges with this? Or do you specifically need the current position of textarea content?