[netfire]

Github claims that the flash version is "currently the only reliable way to put data in the clipboard". This seems to indicate that they looked at the native browser option and found it unacceptable for their use case.

https://github.com/blog/1365-a-more-transparent-clipboard-bu...

[windsurfer]

That blog is from 2013. Github now does browser sniffing to see if it can use native APIs instead of flash: https://gist.github.com/jonrohan/81085b119d16cdd7868a