Insert a paragraph into a list that uses the whole text width

Question

I want to insert a paragraph or possibly even a box with a background color into a list that uses the whole \textwidth. Something like this:

1st item 
   a subitem 1
   b subitem 2

2nd item
   a subitee

\begin{use the full width}
   [ Here the text should use the full text width ignoring the list indent]
\end{use the full width}
   b subitem
3rd item

The list is a normal enumerate.

Answer 1

If you want a oneliner a very simple command is

\makeatletter
\newcommand{\shiftleft}{\hspace*{-\@totalleftmargin}}
\makeatother

If you want an environment that takes up the whole width and breaks accross pages define a list

\makeatletter
\newenvironment{fullwidth}
    {\par
     \setlength{\@totalleftmargin}{0pt}%
     \setlength{\linewidth}{\hsize}%
     \list{}{\setlength{\leftmargin}{0pt}}
     \item\relax}
    {\endlist}
\makeatother

Look at example below:

\documentclass{article}
\usepackage{calc}
\usepackage{color}
\definecolor{shadecolor}{rgb}{1,1,0}

\makeatletter
\newcommand{\shiftleft}{\hspace*{-\@totalleftmargin}}
\newenvironment{fullwidth}
    {\par
     \setlength{\@totalleftmargin}{0pt}%
     \setlength{\linewidth}{\hsize}%
     \list{}{\setlength{\leftmargin}{0pt}}
     \item\relax}
    {\endlist}
%% colored minipage (do not break across pages!)
\newsavebox{\cminibox}
\newlength{\cminilength}
\newenvironment{cminipage}[1][\linewidth]
    {\setlength{\cminilength}{#1-2\fboxsep}
     \begin{lrbox}{\cminibox}%
     \begin{minipage}{\cminilength}}%
    {\end{minipage}\end{lrbox}%
     \colorbox{shadecolor}{\usebox{\cminibox}}}
\makeatother
\begin{document}
\noindent\hrulefill\par
\begin{enumerate}
    \item Item
    \begin{enumerate}
        \item subitem 1

        \shiftleft\colorbox{shadecolor}{A one liner here}

        \item subitem 2
    \end{enumerate}

    \item Item
    \begin{enumerate}
        \item subitem 1

        \begin{fullwidth}
        \begin{cminipage}
        This is a long sentence to test if it fills the
        whole line again and again and again.
        This is a long sentence to test if it fills the
        whole line again and again and again.
        \end{cminipage}
        \end{fullwidth}

        \item subitem 2
    \end{enumerate}
    \item Item
\end{enumerate}
\noindent\hrulefill\par
\end{document}

Answer 2

You can use the \rlap command; in the following example I used this idea and the shaded environment form the framed package to define a command to achieve what you want:

\documentclass{article}
\usepackage{xcolor}
\usepackage{framed}

\colorlet{shadecolor}{blue!40}

\newcommand\MyInd[1]{%
\begin{shaded}\rlap{\parbox{\textwidth}{#1}}\end{shaded}}

\begin{document}

\begin{enumerate}
  \item 1st item 
  \begin{enumerate}
     \item  a subitem 1
     \MyInd{Here the text is using the full text width ignoring the list indent.}
     \item b subitem 2
  \end{enumerate}
  \item 2nd item 
  \begin{enumerate}
     \item  a subitem 1
     \par\rlap{Here the text is using the full text width ignoring the list indent.}
     \item b subitem 2
  \end{enumerate}
  \item 3rd item 
\end{enumerate}

\end{document}

EDIT: the result: