In Opera, on some sites, pressing both mouse buttons simultaneously leads you to some other page, for example, to the separate comments page, when watching a Youtube video. Or to the sitemap on some other websites. May be it is related to Mouse chording ? or is it something different ?
How to control this event (disable it or change the destination page) ?
This is one of Opera's shortcuts for quick back/forward navigation. Holding left button and clicking right takes you forward, holding right and clicking left takes you back.
Opera also uses some heuristics to try to guess what the best 'next' link is if a user tries to go 'forward' but there is no page in the navigation history to go forward to. To control this, you can for example use a rel=next LINK tag to point to the next page. This is most convenient if you have, say, an article split across several pages. On the first page you could put
<link rel="next" href="secondPageOfArticle.htm">
To indicate to the browsers (and search engines) what the next part of the article is.