Making a header width cover whole page

Question

I need to have a header for even and odd pages like this:

I have done this so far:

% !TEX TS-program = xelatex
% !TEX encoding = UTF-8
\documentclass{biditufte-book}
\usepackage{ptext}
\usepackage{xcolor}

\usepackage{showframe}

\usepackage{fancyhdr} % Required for header and footer configuration

\pagestyle{fancy}
\renewcommand{\chaptermark}[1]{\markboth{\normalsize\bfseries #1}{}} % Chapter text font settings
\renewcommand{\sectionmark}[1]{\markright{\normalsize\bfseries #1}{}} % Section text font settings
\renewcommand{\headrulewidth}{0pt}
\setlength{\headheight}{16.5pt}% fancyhdr tells you the length

\newlength{\bluerulelength}
\setlength{\bluerulelength}{\textwidth}
\addtolength{\bluerulelength}{\marginparsep}
\addtolength{\bluerulelength}{\marginparwidth}

\newlength{\pagenumberboxskip}
\setlength{\pagenumberboxskip}{-\bluerulelength}
\addtolength{\pagenumberboxskip}{1cm}
\addtolength{\pagenumberboxskip}{10pt}

\fancyhf{}
\fancyhead[LE]{%
    \small
    \makebox[-5pt][r]{ \pagenumbox{1cm}}%
    \pagehead{\quad\quad\quad\quad\nouppercase{\leftmark}\hfill}%
}
\fancyhead[LO]{%
    \small
    \makebox[\pagenumberboxskip][r]{ \pagenumbox{1cm}}%
    \pagehead{\hfill\nouppercase{\rightmark}\quad\quad\quad\quad}%
}

\newcommand{\pagenumbox}[1]{%
    \begingroup\fboxsep=0pt
    \colorbox{blue!20}{\pagestrut
        \makebox[#1][c]{\thepage}%
    }%
    \endgroup
}
\newcommand{\pagehead}[1]{%
    \begingroup\fboxsep=0pt
    \colorbox{blue}{\pagestrut
        \makebox[\bluerulelength][s]{#1}%
    }%
    \endgroup
}
\newcommand{\pagestrut}{%
    \vrule width 0pt 
    height 1.5\ht\strutbox 
    depth 1.5\dp\strutbox
}
% Removes the header from odd empty pages at the end of chapters
\makeatletter
\renewcommand{\cleardoublepage}{
    \clearpage\ifodd\c@page\else
    \hbox{}
    \vspace*{\fill}
    \thispagestyle{empty}
    \newpage
    \fi}

\usepackage[localise=on]{xepersian}
\settextfont{Times New Roman}
\setdigitfont{Times New Roman}


\begin{document}
    \chapter{مشاهده‌‌پذیری}
    \ptext
    \section{مقدمه}
    \ptext[1-10]
    \section{مقدمه}
    \ptext[1]
\end{document}

which creates something like this:

But that's not what I want. Can someone help me?

Answer 1

The bidi stuff makes some changes to fancyhdr, to accomodate bidi typesetting. For me it's confusing because I am not familiar with bidi. So for example to offset the headers on the left, normally in fancyhdr you use \fancyhfoffset[L]{some length}. But with the bidi stuff you have to use \fancyhfoffset[R]{some length}. And also how the headers behave is different. Maybe there also L and R are swapped. Anyway I have tried to make a solution, but it contains some lengths that I have found with trial and error, and I really don't know where these numbers come from.

Also I got rid of \bluerulelength and \pagenumberboxskip. I thought the blue box could be made \paperwidth long, because that would fill the whole width of the page, but I had to add 2 cm to get it right. Anyway, here is something that might work. If you know how the various length should be interpreted, please let me know.

\setlength{\headheight}{18pt}% fancyhdr tells you the length

\newcommand{\oddoffset}{81mm} % Why? I don't know
\newcommand{\evenoffset}{26mm}

\fancyhf{}
\AtBeginDocument{%
\fancyhfoffset[RO]{\oddoffset}%
\fancyhfoffset[RE]{\evenoffset}%
}

\fancyhead[LE]{%
    \small
    \pagehead{\hspace*{\evenoffset}\pagenumbox{1cm}\quad\nouppercase{\leftmark}\hfill}%
}
\fancyhead[LO]{%
    \small
    \pagehead{\hfill\nouppercase{\rightmark}\quad\pagenumbox{1cm}\hspace*{42mm}}%
}

\newcommand{\pagenumbox}[1]{%
    \begingroup\fboxsep=0pt
    \colorbox{blue!20}{\pagestrut
        \makebox[#1][c]{\thepage}%
    }%
    \endgroup
}
\newcommand{\pagehead}[1]{%
    \begingroup\fboxsep=0pt
    \hspace*{-\evenoffset}%
    \colorbox{blue}{\pagestrut
        \makebox[\paperwidth+2cm][s]{#1}%
    }%
    \endgroup
}