How to create Separator in notebookbar's extension section


I found the preview image in the link: Development/NotebookBar/Extensions - The Document Foundation Wiki

The picture “Extension Preview in NotebookBar” is the feature what I am looking for.
Howerver I can not find document about how to add separator in the extension section.

I am curious about how to do just like the picture show.
Could someone give me some help?
Thanks a bot.

If you are looking for a way to customize the notebook bar, please read: Using the notebook bar, where you can find:

Thanks for reply. But it’s a little different. LO 7.1 give extension ability to add button to the section in notebookbar. Maybe you can try to install the extension provided by developer: LibreOffice-Extension – Google Drive. You can find the extension section in Tabbed toolbar show the button from these extensions.

But it’s a little different.

Sure - from an extension developers view.