to the top left of the code field there should be a 'copy' button, that will open a lightbox that removes the line numbers for copying.

That's not even a programming question! Might I suggest to read the https://forum.processing.org/topic/where-to-place-threads-and-other-forum-practices article? Thanks.

Note that if I select code in a code section like:

1. void setup()
2. {
3.   size(400, 400);
4.   smooth();
5.   frameRate(30);
6. }

I don't get line numbers, in Firefox at least. What browser do you use?