I am not sure when it started, but having problem with this Calibre docker lately. When double click on the book in the main window, calibre viewer opens in the same main window of Calibre. The problem is there is no control in the viewer to close the viewer and get us back to the main window. So, as soon as I open the viewer, I am stuck with the viewer until I restart the docker. I believe the viewer should open in a new tab.
Standalone installations open the viewer in a separate window. Hence, no problem with them.
Thanks.