TikZ, geometry: Gap over tikzpicture

Question

I choose width=18cm, height=26cm in geometry.sty.
If I set the origin at the page center and positionate a node at (-0.5\textwidth, 0.5\textheight) I get a gap over the tikzpicture.

How to get rid of that gap.

\documentclass[]{scrartcl}
\usepackage[showframe=true, width=18cm, height=26cm]{geometry}
\usepackage{tikz}
\usepackage{tikzpagenodes}
\usetikzlibrary{calc, matrix}
\pagestyle{empty}

\begin{document}
\begin{tikzpicture}[
  shift={(current page.center)},
  overlay,remember picture,
  nodes={inner sep=0pt,outer sep=0pt}
]
  \node[
    anchor=north west, 
    draw, red, 
    minimum width=1.0\textwidth,text width=1.0\textwidth-2mm,
    minimum height=6cm,text depth=6cm-4mm,
    align=left, 
  ] at (-0.5\textwidth, 0.5\textheight) (Textbox) {ABC};

  % Help
  \node[blue] at (0,0) {x};
  \draw[blue] (0,0) -- (0,0.5\textheight);
\end{tikzpicture}
\end{document}

Answer 1

Explanation

You shift the tikz coordinates by paper size (shift={(current page.center)}), but put the node relative to text area size (at (-0.5\textwidth, 0.5\textheight)).

• Since the left and right margin between paper size and text area size is the same (in your example), node Textbox is both horizontally centered relative to paper and text area.
• Since the upper and lower margin between that two sizes are different, node Textbox is not vertically centered relative to text area.

Solution

Package tikzpagenodes provides extra nodes having the size of text area, marginpar area, and header and footer area. The new node current page text area is your help:

\documentclass[]{scrartcl}
\usepackage[showframe=true, width=18cm, height=26cm]{geometry}
\usepackage{tikz}
\usepackage{tikzpagenodes}
\usetikzlibrary{calc, matrix}
\pagestyle{empty}

\begin{document}
\begin{tikzpicture}[
  % replace node "current page"
  shift={(current page text area.center)},
  overlay,remember picture,
  nodes={inner sep=0pt,outer sep=0pt}
]
  \node[
    anchor=north west, 
    draw, red, 
    minimum width=1.0\textwidth,text width=1.0\textwidth-2mm,
    minimum height=6cm,text depth=6cm-4mm,
    align=left, 
  ] at (-0.5\textwidth, 0.5\textheight) (Textbox) {ABC};

  % Help
  \node[blue] at (0,0) {x};
  \draw[blue] (0,0) -- (0,0.5\textheight);
\end{tikzpicture}
\end{document}