In that message output mode, copying with the mouse indeed isn't possible. Only a few paging commands are available there. So, that "feature" doesn't exist yet.
There are several ways to obtain the output. The most generic is via :redir
, e.g. into the unnamed register:
:redir @"
:version
:redir END
:put
For a single (last) error message, you can also directly access this via v:errmsg
:
:put =v:errmsg
In the terminal, you could also disable Vim's mouse integration via :set mouse=
, and then use the terminal's mouse drag to copy the (visible) text.