Great idea! I liked the nested previews approach.

Unfortunately this is also limited by CORS policies, the same as other similar iframe-based addons, which leads to not all pages being available for preview – but I guess this could not be resolved until such a feature will be implemented in the browser itself.

My suggestion would be to show different 'Open in background tab' and 'Open in foreground tab' actions. Also would be great to see this extension open-source, and option to reveal hover tooltip only when holding the modifier key (CTRL or Shift).