Matrix library sometimes puts delimiter to the wrong place

[gabor-braun]

Brief outline of the bug

The TikZ library matix have options "left delimiter" etc to add
delimiters to maticies. In the following example, these are obviously at a position having nothing to do with the matrix.

Minimal working example (MWE)

\documentclass[tikz]{standalone}
\usetikzlibrary{matrix}

\begin{document}
\begin{tikzpicture}[ampersand replacement=\&]
  \draw[->] (0, 0) --
  node[matrix of math nodes, anchor=north,
  left delimiter=(, right delimiter=)]
  {0 \& -1 \\ 1 \& 0 \\}
  (1, 0);
\end{tikzpicture}
\end{document}

[muzimuzhi]

This is the sibling issue of #126, which reported wrong positioning of labels.

Each of labels, pins, and matrix delimiters use option append after command to insert another node at the end of the
"main node". But when a node is placed on a line (e.g., (0,0) -- <node> (1,0)) or curve implicitly (see pgfmanual, sec. 17.9), such node is first collected then rescanned as if it is a label (by setting \tikz@node@is@a@labeltrue). When being treated as a label, node positioning is solely controlled by pos option, which is initialized to pos=.5 at picture beginning, and never by normal at setting.

\draw (0,0) -- node[at={(1,1)}] {x}  (1,0); % this `at={(1,1)}` is ignored

To reenable at setting used by matrix delimiters, locally setting \tikz@node@is@a@labelfalse does the trick, just like how 438758d fixed #126. I've prepared a PR.

PS: Setting pos= also works, which maybe helpful in (tricky) user input.