Why `fill` covers `append after command` elements?

Question

Next code is just a little example to describe the problem. I would like to add some elements (lines or text) inside a node and I would like to do it through node's .style. I've tried with append after command (with and without postaction) which works until a fill option is used in outer node. Then, all added elements are covered with filling color and nothing except node contents survives.

Previous figure shows the problem. I would like to obtain the right result

\node[draw, minimum size=3cm, fill=green] (test) {test};
\node[anchor=north] at (test.north) {A};

but using the code used in left node

\node[draw, minimum size=3cm, append after command={%
    \pgfextra \node[anchor=north] at (\tikzlastnode.north) {A};\endpgfextra}] {test};

which produces the central figure when filled

\node[draw, minimum size=3cm, fill=green, append after command={%
    \pgfextra \node[anchor=north] at (\tikzlastnode.north) {A};\endpgfextra}] {test};

I know I could do the same with pic but I would like to understand what happens here. I tried using postaction={append after ..., behind path and in front of path options but always with similar results.

Could you explain it?

\documentclass[tikz,border=2mm]{standalone}

\begin{document}
\begin{tikzpicture}

%Left figure: Works without `fill`.
\begin{scope}[xshift=-4cm]
\node[draw, minimum size=3cm, append after command={\pgfextra \node[anchor=north] at (\tikzlastnode.north) {A};\endpgfextra}] {test};
\end{scope}

%Center figure. Fill covers everything inside
\node[draw, minimum size=3cm, fill=green, append after command={\pgfextra \node[anchor=north] at (\tikzlastnode.north) {A};\endpgfextra}] {test};

%Right figure: two commands
\begin{scope}[xshift=4cm]
\node[draw, minimum size=3cm, fill=green] (test) {test};
\node[anchor=north] at (test.north) {A};
\end{scope}
\end{tikzpicture}
\end{document}

Update (2018)

The main reason of this question was that I wanted to add labels inside node's margins and I didn't know how to do it. I tested all mentioned options I didn't succeed.

Although percusse's answer solved my question, it didn't solve my real problem. So I think it's worth to show the solution here.

Now I know that a label can be drawn inside a node just changing its default anchor. Therefore, the right square can be obtained with a simple command like:

\node[draw, minimum size=3cm, fill=green, label={[anchor=north]A}] (test) {test};

There is no need for extra paths, or append after command and path picture options.

Answer 1

Late options don't cover every property. The append after command is a clever shortcut mechanism for \pgfpositionnodelater<...> commands (here in particular \pgfpositionnodelaterpath). Hence the fill has been already stored but not executed. However the shape etc. has been decided otherwise you wouldn't be able to use that trick for placements. After the placement and other details then \pgfpositionnodenow is invoked and it overprints whatever there was before it.

You can argue maybe the drawing/filling/clipping might have been separated but that would be a feature request for T.Tantau.

Answer 2

I'm really indebted to @Zarko for drawing this question to my attention in this post. My proposal is to draw things on the foreground layer.

\documentclass[tikz,border=2mm]{standalone}
\usetikzlibrary{backgrounds}
\pgfdeclarelayer{background}
\pgfdeclarelayer{foreground}
\pgfsetlayers{background,main,foreground}
\begin{document}
\begin{tikzpicture}

%Left figure: Works without `fill`.
\begin{scope}[xshift=-4cm]
\node[draw, minimum size=3cm, append after command={\pgfextra \node[anchor=north] at (\tikzlastnode.north) {A};\endpgfextra}] {test};
\end{scope}

%Center figure. Fill covers everything inside
\node[draw, minimum size=3cm, fill=green, append after command={\pgfextra{
\begin{pgfonlayer}{foreground} 
\node[anchor=north] at (\tikzlastnode.north) {A};
\end{pgfonlayer}}}] {test};

%Right figure: two commands
\begin{scope}[xshift=4cm]
\node[draw, minimum size=3cm, fill=green] (test) {test};
\node[anchor=north] at (test.north) {A};
\end{scope}
\end{tikzpicture}
\end{document}