How to protrude the left side of a bibliography?

Question

In some circumstances it is very nice to have a bibliography text typeset with protrusion enabled, especially when one uses a bibliography style in which a lot of lines begin with italics. However, the left side of a typical biblatex-generated bibliography doesn't protrude. MWE:

\documentclass[12pt]{article}
\usepackage[style=authoryear]{biblatex}
\usepackage[factor=6000]{microtype}
\usepackage{showframe}
\renewcommand*\ShowFrameLinethickness{.25pt}
\renewcommand*\ShowFrameColor{\color{green}}
\begin{filecontents}{\jobname.bib}
@BOOK{book1,
    AUTHOR = {\char"200B Author, A.},
    TITLE = {Title},
    LOCATION = {Location},
    YEAR = {2017}}
@BOOK{book2,
    AUTHOR = {Author, B.},
    TITLE = {Title},
    LOCATION = {Location},
    YEAR = {2017}}
\end{filecontents}
\addbibresource{\jobname.bib}
\renewcommand{\section}[2]{}
\begin{document}
\noindent Author\par
\nocite{book1,book2}
\printbibliography
\end{document}

which gives:

A quick look at the biblatex.sty reveals that \def\blx@bibitem on line 7746, using the default bibenvironment

\defbibenvironment{bibliography}
  {\list{}{%
     \leftmargin\bibhang
     \itemindent-\leftmargin
     \itemsep\bibitemsep
     \parsep\bibparsep}}
  {\endlist}
  {\item}

creates a \list and places an \item at the beginning of every line. Of course, any character after \item will not protrude to the left.

The pdfTeX manual (p. 34) states:

If you want to protrude some item other than a character (e.g. a \hbox ), you can do so by padding the item with an invisible zero--width character, for which protrusion is activated.

However, I don't want to protrude \items, but all characters differently according to their respective protrusion values.

So, I would be very grateful for any real workaround.

Answer 1

As mentioned in my comment one can get correct protrusion after an \item with the \protrudeleft command describe in Microtype and quote environment leads to different indent of first line.

But it is rather difficult to use this in an automatic environment like the bibliography. At first the command needs at least the first char as argument, so with pdflatex and utf8 something like \protrudeleft Ä won't work as if would use only the first half of the Ä as argument, one need braces \protrudeleft{Ä} -- with lualatex or xelatex it would be easier, but even there one can't be sure that the first input token is really a char. But beside this: biblatex is doing so many things at the begin of an entry that it is very difficult (or even impossible) to inject such a low level command.

An more easier alternative is to try to avoid the use of a list for the bibliography. This works quite ok (the indent of the T from Title is due to the large microtype settings):

\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[style=authoryear]{biblatex}
\usepackage[factor=6000]{microtype}
\usepackage{showframe,xpatch}
\renewcommand*\ShowFrameLinethickness{.25pt}
\renewcommand*\ShowFrameColor{\color{green}}

\addbibresource{test.bib}
\renewcommand{\section}[2]{}


\defbibenvironment{bibliography}
  {\parindent=0pt 
   \bibitemsep=1ex
   \endgraf}
  {\endgraf}
  {\endgraf\vspace{\bibitemsep} \hangindent=\bibhang \hangafter=1}

\xapptocmd\finentrypunct{\par}{}{\fail} %the paragraph musst be closed before the \endgroup
\textwidth=3cm  
\begin{document}

\noindent Author\par
\noindent Äuthor\par

\nocite{book1,book2}

\printbibliography
\end{document}