How to exit Calibre reader

Probably, a dumb question.

I am using linuxserver docker of Calibre server (4.3).

When I double click on a book, Calibre reader opens and loads the book. So far OK.

But, I don't know how close the reader and get back to the Calibre main window.

The only fix I found: Make the reader enter full screen. Then again exit full screen. The reader window becomes small and I can see the main Calibre window behind it. When I click anywhere on the main window, the reader window disappears.

Must be an easy way.

Thanks.