The copy button on assistant messages was always visible on mobile
(opacity-60), positioned as a detached row below the message content.
Hide it entirely on small screens (hidden sm:flex) and keep hover-only
behavior on desktop. Mobile users can still copy via native long-press
text selection.
via [HAPI](https://hapi.run)
Co-authored-by: HAPI <noreply@hapi.run>