`sprint(; context=io)` loses `displaysize` of `io`

[KristofferC]

julia> displaysize(stdout)
(17, 160)

julia> displaysize(IOContext(stdout))
(17, 160)

julia> sprint(; context=stdout) do io
           print(io, displaysize(io))
       end
"(24, 80)"

Imo, the last one should be consistent with the others.

Workaround:

julia> ioc = IOContext(stdout, :displaysize=>displaysize(stdout));

julia> sprint(; context=ioc) do io
           print(io, displaysize(io))
       end
"(17, 160)"

[vtjnash]

This is mainly intentional, though a frequently asked question. The rationale being that even if the terminal was that size, the portion of the terminal relevant to the inner print is only a subset of that size, so we instead just use the default value.

[KristofferC]

the portion of the terminal relevant to the inner print is only a subset of that size, so we instead just use the default value.

I don't understand this. Why would the default value ever be more useful when it bears no relation to the terminal at all over the actual value of the context that gets passed to sprint?

I think a standard use-case for sprint is to buffer the printout to a terminal to do the final print in one call. The current behavior makes that awkward.

[jmichel7]

If an IOContext loses its :displaysize in some circumstances, this should be documented.

[vtjnash]

Duplicate of #34721