Steve, I am using Opera latest version. 3 systems same results, and no matter about resolution. This issue is related to Opera only. Chrome and Firefox latest versions, no issue.
1) Go here:
http://demo.x-cart.com/demo/Three-Stone-Princess-Cut-Diamond-Ring.html
2) Click on "Customer also bought" tab. Now scroll the page down with your mouse wheel. Look at the bottom, a white space appears bellow footer.
3) Now click on Customer feedback. Scroll again, the white space bellow footer is still there. Try click on Send to Friend, the layout moves a little bit up, mouse pointer also, but Send to Friend tab is not active
The key of finding this issue is scrolling down the page at maximum with different tabs. You can see the screenshot for mysterious appearing/disappearing white space bellow the footer as attachment. If you did not find this issue, I will make a movie for this behavior.