As it turns out, the Navigation-Popups-Gadget gadget does not only conflict with Page-Previews, but also with Reference Previews. The user experience is fine: the Popups extension (including reference previews) is entirely disabled when the gadget is enabled.

However: it feels like it would make sense to use the Navigation-Popups-Gadget gadget and Reference Previews the same time. This would require some flag to disable Navigation-Popups-Gadget only for references. Is this something we want? Should we ask the gadget devs to either provide a setting for this?