I'm pretty sure that this wasn't happening with the 9.x releases. But, I found that when I middle-clicked in Opera to get the scroll icon and scroll up/down a page, all that happened was that whatever was in my clipboard got pasted into the page, with the page trying to navigate to the clipboard text. In Opera's defence, the paste action is generally the default action across all applications in Linux. However, this also made this behaviour inconsistent across different platforms and even across other browsers on the same platform.
While I could not find any relevant options in the Opera preferences pages, I finally found out today how to customise the middle-click. Obscurely enough, the preferences page for the middle-click is accessed by "shift-middle-clicking". This brings up a dialogue which offers choices on what will happen when you middle-click a blank part of a page and similarly for hyperlinks.
All very simple, yet rather inaccessible. But all said and done, I'm a happy chappy now.