After a design is complete or during the process when you click preview it opens in the same window. There is a small X in the top right corner that can be missed easily. If you click on the back button at that point you lose your design.
Can it be set to open in a lightbox to preview the design or at least a new window or tab?
I'm using the wordpress version.
The preview already displays in a lightbox (fullscreen). With the plugin as it is the only thing you could do is add some custom CSS (UI & Layout Composer, custom CSS tab) to change the dimensions and position, this class should be the one you need: fpd-modal-internal. This will also affect the lightbox display mode!