Not sure overwriting the user's clipboard is quite appropriate.

The clipboard is only overwritten when the user clicks on the result. It is quite likely that if someone is doing quick math it is because they wish to use the result. There is no other way to use the result than copying it to the clipboard. If you can think of a better method, I would be happy to implement it that way. As it is, I see little harm in this.

This has been sitting here for a while, with no further comments have been made. Personally, I believe that copying to clipboard on click makes sense, but in any case, this is an extensions: design issues should not block it. So, I'd say approved.