You can attach the Mailbutler Sidebar to your Apple Mail window. Or you can detach it to have a floating sidebar.

To attach the Mailbutler Sidebar click on the attach button.

Once you have attached the Mailbutler Sidebar, it will stay attached by default.

To detach the Mailbutler Sidebar, click the attach button again.

For instructions on how to open/close the Mailbutler Sidebar on macOS Sonoma, head to this article.