\x ref argument

The ref argument of \x marks the link as reference, e.g.:renders something like:

Without content, clicking the reference navigates directly to its target. When content is given, clicking opens a small popover beside the reference instead:The popover starts with the normal linked target title, followed by ". " and the content rendered as regular OurBigBook markup, including links, formatting and paragraphs. For \a ref argument, it starts with the linked URL instead. Click outside the box or press Escape to close it. References without content (including an empty content argument) remain direct links.

Implemented at: github.com/ourbigbook/ourbigbook/issues/137