I had a similar experience in Chrome, pasting into gedit. Ctrl-C/Ctrl-V works, but menu copy/paste doesn't.

If you read the source of the website, it has a

> document.addEventListener('keydown', function(event) { ... })

Basically, press any key on your keyboard on the website, and it'll work.

You do have to use the keyboard shortcut. It executes the snippet regardless of which key sequence you press though ('keydown' event on document).

It may also be because you have no support for execCommand if it didn't work for CTRL+C.