" If I click the map in that pop-up it opens a new tab on the page below it "
Yes, that is the problem. In all browsers, except Opera, the map appears, as should be. on top. After all, if you pressed the map link, that's because you want to see it, not having it partly hidden.
How can I get round this? Is there a contact for the Opera Developers for them to see if this is a bug?