I often post a comment with ⌘+↵ and then try go to the notifications or PR list with a keyboard shortcut - e.g. g,n.

However, the comment field is still focused after posting my comment and I just end up typing gn in the textarea.

I suggest to trigger blur (unfocus) on tne textarea after successful submission so keyboard shortcuts on GitHub work immediately.

I don't think posting subsequent comments is often enough to be a problem.

An alternative approach which I also like is to map Esc to blurring the textarea.