How can I position elements absolutely on a page?

Question

I am trying to position elements absolutely on a page. These positioned elements use the put{} command itself. The coordinate origin should always be in the top left corner on every page, also when changing the page layout (by using \newgeometry{}).

---

To achieve this I want to use the second parameter set of the picture environment. This should translate my coordinate system to the top left corner of the paper.

The position of the picture environment is defined by the position of the header and/or the page layout. So the second set of parameters should depend on any or multiple of the page margins. When the page changes the margins will change and my origin will stay the same.

So in other words: I am trying to calculate back the position of the header/picture-environment. When I know how this position is calculated relative to the top left corner I can calculate back the position from inside the header to the top left paper corner.

For setting the elements I tried to use the fancyhdr package together with geometry and including a picture environment. But I could not figure out how the position of the picture is being calculated. Something like this (whole code and result shown below):

\fancyhead[L]{
    \begin{minipage}{\textwidth}
        \headertext \\
        \begin{picture}(0,0)(\geometryleft,-\geometrytop)
            \put(0, 0){\fbox{fancyhdr}}
        \end{picture}
    \end{minipage}
}

But when I change the page layout by using \newgeometry the element changes its position. The \geometryleft works. After reading the geometry documentation I modified the \geometrytop but I was not able to find the correct calculation. According to the docs I was expecting that the correct term is

\top - \headsep - \layoutvoffset.

But this is not correct. (For a better clearness I only wrote \geometrytop here.)

Also I tried to use the everypage package because I have to include it anyway. It seems like the everypage version positions the elements indipendently of the geometry package but still I could not figure out how to calculate the correct 0/0-point to set it in the top left corner. The short code looks something like this (whole code and result shown below):

\AddEverypageHook{
    \begin{picture}(0,0)(\hoffset+1in,-\voffset-1in)
        \put(0, 0){fbox{everypage}}
    \end{picture}
}

Because the layout changing of the geometry package are completely ignored I was expecting that the everypage version uses the (La-?)TeX defaults. I found them to be \hoffset + 1 in for the left margin and \voffset + 1 in for the top offset. As in the fancyhdr version the left offset works, the right one is wrong.

Now my question is: How do I get the correct distances from the top and left of the paper to the picture-environment position? Which version I use does not matter. If you know another possibility (without including an additional pacakge if possible) I would also appreciate it.

Note that I want to avoid using tikz. Also I am not placing simple boxes but I am "calling" definded commands. Those commands contain the put{} command itself. It is very important for them that the coordinate origin will always be the same.

A working minimal example can be found below the image. The results of both versions are shown in the image below. There are boxes (in the top left corner). Both of them should exactly be in the corner on every page, no matter how the layout is changed.

The full code is:

\documentclass[a4paper]{article}

\newlength{\geometryleft}
\setlength{\geometryleft}{2cm}
\newlength{\geometryright}
\setlength{\geometryright}{2cm}
\newlength{\geometrytop}
\setlength{\geometrytop}{5.5cm}
\newlength{\geometrybottom}
\setlength{\geometrybottom}{1cm}
\newlength{\geometryheadheight}
\setlength{\geometryheadheight}{5.5cm}

\usepackage[
    showframe,
    left=\geometryleft, 
    right=\geometryright, 
    top=\geometrytop, 
    bottom=\geometrybottom, 
    headheight=\geometryheadheight
]{geometry}
\usepackage{fancyhdr}
\usepackage{picture}
\usepackage{calc}
\usepackage{color}
\usepackage{everypage}

\newcommand{\headertext}{Text of the first header}

\fancypagestyle{fancyhdr}{
    \renewcommand{\headrule}{%
        \hbox to\textwidth{%
            \color{black}\leaders\hrule height .4pt\hfill%
        }%
    }
    \renewcommand{\footrule}{%
        \hbox to\textwidth{%
            \color{black}\leaders\hrule height .4pt\hfill%
        }%
    }

    \fancyhead[L]{%
        \begin{minipage}{\textwidth}
            \headertext \\
            \begin{picture}(0,0)(\geometryleft,-\geometrytop+3cm)
                \fbox{fancyhdr}
            \end{picture}
        \end{minipage}
    }
}

\pagestyle{fancyhdr}

\AddEverypageHook{
    \begin{picture}(0,0)(\hoffset+1in,-\voffset-1in)
    \fbox{everypage}
    \end{picture}
}

\begin{document}
    Text 
    
    \clearpage
    \renewcommand{\headertext}{
        A lot longer header text \\
        with \\
        multiple \\
        lines \\
        so \\
        it \\ 
        is \\ 
        higher
    }
    Text
    
    \clearpage
    \setlength{\geometryheadheight}{2cm}
    \newgeometry{
        headheight=\geometryheadheight,
        top=\geometrytop,
        left=\geometryleft,
        right=\geometryright
    }
    Text
    
    \clearpage
    \setlength{\geometryleft}{5cm}
    \setlength{\geometryright}{.5cm}
    \setlength{\geometrytop}{.5cm}
    \newgeometry{
        headheight=\geometryheadheight,
        top=\geometrytop,
        left=\geometryleft
        ,right=\geometryright
    }
    Text
\end{document}

---

Why @David Carlisle answer is not working

The code below shows why the code of @David Carlisle is not working. My problem is, that the page dimension is changing in the document but my commands (\fancycommand) do not know how they are changing. The following code shows his code in my minimal example:

\documentclass[a4paper]{article}

\newlength{\geometryleft}
\setlength{\geometryleft}{2cm}
\newlength{\geometryright}
\setlength{\geometryright}{2cm}
\newlength{\geometrytop}
\setlength{\geometrytop}{5.5cm}
\newlength{\geometrybottom}
\setlength{\geometrybottom}{1cm}
\newlength{\geometryheadheight}
\setlength{\geometryheadheight}{5.5cm}

\usepackage[
    showframe,
    left=\geometryleft, 
    right=\geometryright, 
    top=\geometrytop, 
    bottom=\geometrybottom, 
    headheight=\geometryheadheight
]{geometry}
\usepackage{fancyhdr}
\usepackage{picture}
\usepackage{calc}
\usepackage{color}
\usepackage{everypage}

\newcommand{\headertext}{Text of the first header}

\fancypagestyle{fancyhdr}{
    \fancyhead[L]{%
        \begin{picture}(0,0)
            \put(-57,117){\fancycommand}

            % this is only included to show that the fancytext is
            % still displayed and that the position changes
            \put(0,0){\fancycommand}
        \end{picture}%
        \headertext
    }
}

\newcommand{\fancycommand}{\put(10, -10){\color{red}Fancy text}}

\pagestyle{fancyhdr}

\begin{document}
    Text 
    
    \clearpage
    \renewcommand{\headertext}{
        A lot longer header text \\
        with \\
        multiple \\
        lines \\
        so \\
        it \\ 
        is \\ 
        higher
    }
    Text
    
    \clearpage
    \setlength{\geometryheadheight}{2cm}
    \newgeometry{
        headheight=\geometryheadheight,
        top=\geometrytop,
        left=\geometryleft,
        right=\geometryright
    }
    Text
    
    \clearpage
    \setlength{\geometryleft}{5cm}
    \setlength{\geometryright}{.5cm}
    \setlength{\geometrytop}{.5cm}
    \newgeometry{
        headheight=\geometryheadheight,
        top=\geometrytop,
        left=\geometryleft
        ,right=\geometryright
    }
    Text
\end{document}

In the result you can see that on page one (left) the red "Fancy text" is displayed perfectly on the top left corner of the page. On the second page this is not the case anymore. This is because the layout changed. The problem is, that the position in the fancyhdr put{} is hard coded here. This has to be dependent on the page layout. Exactly this is my question: How the position (no matter if the first put or the origin of the picture environment) depends on the page margins.

The \fancycommand should only set its coordinate relative to the complete page. It should not be necessary to check the page dimensions. So the \fancycommand uses the full page as a coordinate system. I hope this helps to clarify my question.

Answer 1

It's much easier using eso-pic's \AtPageUpperLeft to place content at the upper-left of the page:

\usepackage{eso-pic}
\AddToShipoutPictureFG{
  \AtPageUpperLeft{%
    \raisebox{-\height}{%
      \fbox{everypage}%
    }%
  }%
}

Since content is usually placed on the baseline, \raisebox{-\height}{<stuff>} ensures that the top of <stuff> will be at the top of the page inside \AtPageUpperLeft.

Answer 2

beware adding spaces and also picture mode content should always be in \put so

\fancyhead[L]{%
        \begin{picture}(0,0)
            \put(10,-200){\fbox{fancyhdr}}
        \end{picture}%
    \headertext
}

would put the \fbox{fanchdr} at 10,-200 relative to the left edge of the page header, you can adjust the coordinates to place it anywhere on the page.