[rashkae]

Kobo documentation says Kobo supports Epub3 files. However, I have found that in recent firmware versions, (since 4.14), Kobo does not use the epub3 nax.xhtml file, but rather the ncx file, (if any) as the table of contents. In the absense of ncx file, epub3 files have no software TOC. (Inline / in book TOCs, of course, still work, though there is no means to get to one quickly.)

I'm not sure if this was always the case, or if oder version of calibre used to include both ncx and nav toc files when producing epub3 files. I did not notice this issue back with firwmare 4.12