[Tex/LaTex] How to calculate width of remaining part of line

Question

I wish to create section heading using titlesec like image

But I have some difficulties in determining the width of remaining of line to include blue box containing section label, my method consist of subtracting length of gray box from textwidth but it did not work.

\documentclass[a4paper]{book}
\usepackage{lipsum}
\usepackage[explicit]{titlesec}
\usepackage{xcolor}

\newlength{\myl}
\colorlet{mygray}{gray!90}
\colorlet{myblue}{blue!80}

\newcommand{\graybox}{\colorbox{mygray}{\strut \color{white}Section~\thesection}}

\settowidth{\myl}{\graybox}


\titleformat{\section}[hang]{\large\bfseries}%
{\graybox}{.5ex}{\colorbox{myblue}{\makebox[\dimexpr \linewidth-\myl-2\fboxsep-.5ex][l]%
{\strut \color{white}\large\bfseries #1}}}

\pagestyle{empty}

\begin{document}

Answer 1

The computations are somewhat similar to those in the other answers, but this solution also copes with unnumbered sections and long titles.

\documentclass[a4paper]{book}
\usepackage{lipsum}
\usepackage{titlesec}
\usepackage{xcolor}

\newsavebox{\sectionlabelbox}
\newlength{\sectionlabelwidth}
\colorlet{mygray}{gray!90}
\colorlet{myblue}{blue!80}

\newcommand{\sectionlabel}{%
  \sbox{\sectionlabelbox}{\colorbox{mygray}{\strut\color{white}Section~\thesection}}%
  \global\sectionlabelwidth=\wd\sectionlabelbox
  \usebox{\sectionlabelbox}%
}
\newcommand{\sectiontitle}[1]{%
  \colorbox{myblue}{%
    \parbox[t]{\dimexpr\columnwidth-\sectionlabelwidth-2\fboxsep-0.5ex}{
      \raggedright\strut\color{white}#1
    }%
  }%
}

\titleformat{\section}[hang]
  {\large\bfseries\global\sectionlabelwidth=-0.5ex }%
  {\sectionlabel}
  {.5ex}
  {\sectiontitle}

\begin{document}

\section{A test}

\setcounter{section}{9}

\section{Another test}

\section{Another test, but with a title that is so long it has to be
split across lines}

\section*{A further test}

\end{document}

Here's a modification where the grey box has the same vertical size of the blue box.

\documentclass[a4paper]{book}
\usepackage{lipsum}
\usepackage{titlesec}
\usepackage{xcolor}

\newsavebox{\sectiontitlebox}
\newlength{\sectionlabelwidth}
\colorlet{mygray}{gray!90}
\colorlet{myblue}{blue!80}

\newcommand{\sectiontitle}[1]{%
  \settowidth{\sectionlabelwidth}{%
    \colorbox{mygray}{\strut\color{white}Section~\thesection}%
  }%
  \sbox{\sectiontitlebox}{%
    \colorbox{myblue}{%
      \parbox[t]{\dimexpr\columnwidth-\sectionlabelwidth-2\fboxsep-0.5ex}{
        \raggedright\strut\color{white}#1
      }%
    }%
  }%
  \colorbox{mygray}{%
    \vrule height \dimexpr\ht\sectiontitlebox-\fboxsep\relax
           depth  \dimexpr\dp\sectiontitlebox-\fboxsep\relax
           width 0pt
    \color{white}Section~\thesection
  }%
  \hspace{0.5ex}%
  \usebox{\sectiontitlebox}%
}

\newcommand{\sectionstartitle}[1]{%
  \colorbox{myblue}{%
    \parbox[t]{\dimexpr\columnwidth-2\fboxsep}{
        \raggedright\strut\color{white}#1
    }%
  }%
}

\titleformat{name=\section}[hang]
  {\large\bfseries}
  {}
  {0pt}
  {\sectiontitle}
\titleformat{name=\section,numberless}[hang]
  {\large\bfseries}
  {}
  {0pt}
  {\sectionstartitle}

\begin{document}

\section{A test}

\setcounter{section}{9}

\section{Another test}

\section{Another test, but with a title that is so long it has to be
split across lines}

\section*{A further test}

\end{document}