Description
As already discussed in #1593, linear=no makes the <itemref> auxiliary. Breaking the linearity of an <itemref> allows the reading system to present its content in a different order from that one specified in the <spine>.
From the ePub 2 spec:
Specifying whether an OPS Content Document is primary or auxiliary is useful for Reading Systems which opt to present auxiliary content differently than primary content. For example, a Reading System might opt to render auxiliary content in a popup window apart from the main window which presents the primary content. (For an example of the types of content that may be considered auxiliary, refer to the example below and the subsequent discussion.)
Reading Systems are not required to differentiate between primary and auxiliary content, and for the requirements and recommendations given in this section may consider all OPS Content Documents in
spineto be primary, regardless of the value of thelinearattribute.
Or the ePub 3 spec:
The
itemrefelementlinearattribute indicates whether referenced item is considered primary (yes) or auxiliary (no) in thespine. This attribute may be used to enable Reading Systems to distinguish presentation of body content from supplementary content which might be, for example, presented in a popup window or omitted from an aural rendition.
From both descriptions, I think it is important to remove linear=no from <itemref idref="cover" linear="no" /> in both ePub 2 & 3 formats.
As far as I understand, the book cover is no axiliary content. Either way, it shouldn’t be presented in a popup window.
Many thanks for your help.