This doesn't happen in the Calibre Editor preview or on my Kobo Forma. Only in the Calibre Viewer. I was testing margins and set up a fake book with 5em margins all around. In the Viewer, the text pays no attention to the right margin. It just keeps going. I've included a link to my fake book in case anyone wants to see if I've messed up any particular settings:
And, the book is here:
https://drive.google.com/file/d/1NdG...ew?usp=sharing
.
EDIT: Sorry. I'm on a fully updated Windows 11 Pro x64 and Calibre 6.3. And, I don't see that odd behavior in books with more normal (0 or 0.5em) margins.