This is a cousin to https://gist.github.com/jfbu/2f3d04414359f716f98ea64dac91eb0e

fullwidthspaceissue.tex

\documentclass{article}

\newenvironment{foo}{}{}

\begin{document}

\begin{foo}
\noindent\begin{minipage}{\linewidth}
  bar!
\end{minipage}
\end{foo}

There is blank line above me.
\end{document}

@davidcarlisle, thanks. Indeed pinging does not work and I don't get pings from your comments here either.

I see now clearer: with the -1sp, the first space remains at end of line (creating an overfull hbox), which is not as bad as moving it to next line, as it would then create a way underfull last line. (an empty one actually as the space is removed, being at its start)

With the full width, there is a way underfull (empty) last line. As the \unskip removed only one space, the first one makes TeX start a new line even though it will be discarded next.

Well this is very surprising nevertheless, because one could have thought that it was perfectly legitimate too to leave a trailing space at end of full last line, making it slightly overfull. This does look to me as a deficiency of TeX's handling of spaces at end of a paragraph, but I have not digged into tex.web