How to calibrate fancy headers?

Question

I was inspired by this example to try and make some advanced headers using tikz. I attempted to construct a simple example in order to understand how it all works together and managed to create this: The problem is that the header doesn't extend across the entire page like the example, and I've tried to figure out how but to no avail, unfortunately. Here's the MWE:

\documentclass{article}
\usepackage[a4paper, hmargin=2cm, vmargin=1cm, includeheadfoot]{geometry}
\usepackage[nocheck]{fancyhdr}
\usepackage{tikz}
\usetikzlibrary{calc}

\renewcommand{\headrulewidth}{0pt}

\setlength{\headheight}{20pt}

\pagestyle{fancy}

\fancyhead[R]{
    \begin{tikzpicture}
        \draw[left color = red!80!black, right color=red!50!black] (current page.north west) rectangle ($ (current page.north east) +(-4cm,-1cm) $);
        \node[circle, very thick, draw=yellow, text = white] at ($ (current page.north east) + (-4.5cm, -0.5cm) $) {\thepage};
    \end{tikzpicture}
}

\begin{document}

Here's a random introduction

\newpage

Here's some more random text

\end{document}

In the example, the original poster could just use the current page function of tikz to ensure the header stretched across horizontally without further modifications, yet I need to manually do that with the calc library if I want to achieve the same effect. How can I auto calibrate the header to extend across the page horizontally?

Answer 1

I'm not sure, what is your problem. I guess that something like this:

I didn't bother with rectangle color, its height and position. This is easy to add or change in the MWE below:

\documentclass{article}
\usepackage[a4paper, hmargin=2cm, vmargin=1cm, includeheadfoot]{geometry}

\usepackage{tikz}
\usepackage{fancyhdr}
\pagestyle{fancy}
\renewcommand{\headrulewidth}{0pt}
\fancyhead[C]{%
    \begin{tikzpicture}[remember picture, overlay]
\fill[teal]    (current page.north west)rectangle  ([yshift=-1cm] current page.north east);

\node[circle, draw, thick, minimum size=7.5mm, inner sep=0pt, white]
    at ([xshift=-1cm, yshift=-0.5cm] current page.north east)   {\thepage};
    \end{tikzpicture}
            }
\begin{document}
\pagestyle{fancy}

The first page

\newpage

The second page

\end{document}