Lengthen side edges of node

Question

Say I have a simple rectangular node I wish to lengthen some sides of, such that they extend further from the corners. Simple, here is my attempt:

\documentclass[tikz,border=1mm]{standalone}

\begin{document}

\begin{tikzpicture}
    \node[draw] (x) {foo};
    \draw (x.north west) -- ++(0, +1mm);
\end{tikzpicture}

\end{document}

The result:

I see what the problem is: x.north west is exactly at the corner of the shape and draw draws the line from this corner in such a way that the drawn path protrudes by pgflinewidth/2 in both directions along the trajectory I specified (if that makes sense; sorry, it is rather hard to explain).

My question is, how do I draw the line so that it looks like it extends the original side of the x node?

• I suppose the x.north east anchor is where it is, there is no other anchor that just happens to be where I need it
• One thing I thought of is making the draw command \draw[xshift=.5\pgflinewidth], but it did not cause any change and feels a bit hacky

Answer 1

Try

\documentclass[tikz,border=1mm]{standalone}

\begin{document}

\begin{tikzpicture}
    \node[draw,outer sep=0pt] (x) {foo};
    \draw (x.north west) -- ++(0, +1mm);
\end{tikzpicture}

\end{document}

Anchor of nodes is on outside of node's borders, i.e. for outer sep moved from middle of borders lines. This you can change by settings outside sep=0pt

Answer 2

Improving on my attempted ‘hacky’ solution:

As far as I understand, xshift does not apply to points referenced by nodes, in this case (x.north west). Instead, one can use transform canvas, like this:

\documentclass[tikz,border=1mm]{standalone}

\begin{document}

\begin{tikzpicture}
    \node[draw] (x) {foo};
    \draw[transform canvas={xshift=.5\pgflinewidth}] (x.north west) -- ++(0, +1mm);
\end{tikzpicture}

\end{document}

The result:

This ultimately solves my problem, but does not feel quite right. Is there a better way to do this? ☺