Allow environments and cells to render Non-Cell outputs in the notebook.

[pankgeorg]

[github-actions[bot]]

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/fonsp/Pluto.jl", rev="pg/env-sidebar")
julia> using Pluto