@KevinH - I think its Book View + Preview, Code View and Preview together seem to be OK, Book View by itself seems to be OK.
Aside : On Windows if you open EPUBs with Sigil (eg from file manager or calibre), you will get a new Sigil process for each EPUB opened, or if you start Sigil multiple times you will get multiple processes. It's only when you use File->Open that the current process is 'reused'.
BR
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