Ecco qua, gli ho dato una sistemata. Metto un esempio:
`
\documentclass[12pt]{article}
\usepackage[T1]{fontenc}
\usepackage[latin1]{inputenc}
\usepackage{amsmath,amsfonts}
\usepackage{framed,color,lipsum}
\newlength\sidebar
\newlength\envrule
\newlength\envborder
\setlength\sidebar{1.5mm}
\setlength\envrule{0.4pt}
\setlength\envborder{2.5mm}
\makeatletter
\long\def\fboxs#1{%
\leavevmode
\setbox\@tempboxa\hbox{%
\color@begingroup
\kern\fboxsep{#1}\kern\fboxsep
\color@endgroup}%
\@frames@x\relax}
\def\frameboxs{%
\@ifnextchar(%)
\@framepicbox{\@ifnextchar[\@frameboxs\fboxs}}
\def\@frameboxs[#1]{%
\@ifnextchar[%]
{\@iframeboxs[#1]}%
{\@iframeboxs[#1][c]}}
\long\def\@iframeboxs[#1][#2]#3{%
\leavevmode
\@begin@tempboxa\hbox{#3}%
\setlength\@tempdima{#1}%
\setbox\@tempboxa\hb@xt@\@tempdima
{\kern\fboxsep\csname bm@#2\endcsname\kern\fboxsep}%
\@frames@x{\kern-\fboxrule}%
\@end@tempboxa}
\def\@frames@x#1{%
\@tempdima\fboxrule
\advance\@tempdima\fboxsep
\advance\@tempdima\dp\@tempboxa
\hbox{%
\lower\@tempdima\hbox{%
\vbox{%
%\hrule\@height\fboxrule
\hbox{%
\vrule\@width\fboxrule
#1%
\vbox{%
\vskip\fboxsep
\box\@tempboxa
\vskip\fboxsep}%
#1%
}%\vrule\@width\fboxrule}%
}%\hrule\@height\fboxrule}%
}%
}%
}
\def\esefcolorbox#1#{\esecolor@fbox{#1}}
\def\esecolor@fbox#1#2#3{%
\color@b@x{\fboxsep\z@\color#1{#2}\fboxs}{\color#1{#3}}}
\makeatother
\definecolor{exampleborder}{rgb}{0,0,.7}
\definecolor{examplebg}{rgb}{.9,.9,1}
\definecolor{statementborder}{rgb}{.9,0,0}
\definecolor{statementbg}{rgb}{1,.9,.9}
\newenvironment{eseframed}{%
\def\FrameCommand{\fboxrule=\the\sidebar \fboxsep=\the\envborder%
\esefcolorbox{exampleborder}{examplebg}}%
\MakeFramed{\FrameRestore}}%
{\endMakeFramed}
\newcounter{esempio}
\numberwithin{esempio}{section}
\renewcommand{\theesempio}{\thesection.\arabic{esempio}}
\newenvironment{esempio}
{\par\medskip\refstepcounter{esempio}%
\hbox{%
\fboxsep=\the\sidebar\hspace{-\envborder}\hspace{-.5\sidebar}%
\colorbox{exampleborder}{%
\hspace{\envborder}\footnotesize\sffamily\bfseries%
\textcolor{white}{Esercizio\enspace\theesempio\hspace{\envborder}}
}
}
\nointerlineskip\vspace{-\topsep}%
\begin{eseframed}\noindent\ignorespaces%
}
{\end{eseframed}\vspace{-\baselineskip}\medskip}
\begin{document}
\section{sezione}
Testo testo
\begin{esempio}
\lipsum[1]
\end{esempio}
Testo testo testo testo Testo testo testo testo Testo testo testo testo Testo testo testo testo Testo testo testo testo, Testo testo testo testo.
\begin{esempio}
\lipsum
\end{esempio}
\lipsum[2]
\end{document}
`
Nell’esempio ho uato \lipsum e crea un warning overfull vbox. Se non lo usi non si presenta. Comunque ti consiglio di caricare anche il pacchetto microtype.
Ciao
Andrea