May be it is not really a big problem, but it would be a nice addition to the site features if there was a button over or after the code to select all the code. I think it is useful more in cases you use a semi-broken mouse (my case) that forgets that a button is clicked and "unselects" a big part of a big code... And I have to go back to start to select again (Don't ask for my mouse brand because some "Bill" will go after me)...
Independent of my specific problem, in big files it is really useful and many communities use such buttons.