Cell color overriding cell rounded border

Question

In a table, made with Tikz, I tried to make the first row darker than the rest by, I think, a little of brute force. It happens that the borders I want must be rounded and the extreme cells (north west, north east, south east and south west ones) are overring it as this image shows:

(The lighter cyan is a little hard to see, but yes, still overriding the border)

How to solve this problem? Here's my MWE:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix}
\begin{document}
\begin{tikzpicture}[scale=.9]
\matrix[matrix of math nodes,minimum width=2.5cm,minimum height=6mm,column 1/.style={nodes={fill=cyan!7}}, column 2/.style={nodes={fill=cyan!7}}, row 1/.style={nodes={fill=cyan!45}}] (func) {
 x & y = 2x + 1\\
-3&-5\\
-2&-3\\
-1&-1\\
0&1\\
1&3\\
2&5\\
3&7\\

};
\draw [rounded corners=5pt,thick] (func-1-1.north west) rectangle (func-8-2.south east);
\draw [thick] (func-1-1.south west) -- (func-1-2.south east);
\draw [thick] (func-1-1.north east) -- (func-8-1.south east);
\end{tikzpicture}
\end{document}

Answer 1

Got something working for you!

Instead of doing the fill via the matrix command, I did it afterwards, using arcs for the corners.

The code requires \usetikzlibrary{calc} because relative coordinates are calculated. Also, note how I am using layers to have the fill go behind the text of your matrix.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix}
\usetikzlibrary{calc}
\begin{document}
\begin{tikzpicture}[scale=.9]
\pgfdeclarelayer{background}
\pgfsetlayers{background,main}
\matrix[matrix of math nodes,minimum width=2.5cm,minimum height=6mm,column 1/.style={nodes={fill=cyan!7}}, column 2/.style={nodes={fill=cyan!7}}, row 1/.style={nodes={fill=none}}] (func) {
     x & y = 2x + 1\
    -3&-5\
    -2&-3\
    -1&-1\
    0&1\
    1&3\
    2&5\
    3&7\
};
% Fill in top row
\begin{pgfonlayer}{background}
    \filldraw [fill=cyan!45,thick] (func-1-1.south west) -- ($ (func-1-1.north west)+(0,-5pt)$ )
    arc (180:90:5pt)
    -- ($ (func-1-2.north east) + (-5pt,0) $)
    arc (90:0:5pt)
    -- (func-1-2.south east);
\end{pgfonlayer}
\draw [rounded corners=5pt,thick] (func-1-1.north west) rectangle (func-8-2.south east);
\draw [thick] (func-1-1.south west) -- (func-1-2.south east);
\draw [thick] (func-1-1.north east) -- (func-8-1.south east);
\end{tikzpicture}
\end{document}

The path starts at the south-west corner of the first row. Then it goes up to the top-left corner, just 5pt below, which is the radius of your rounded corner. Then, I have the arc of 5pt, and you see how the pattern continues all the way to the south-east corner of the first row.

I must admit I'm not very familiar with tikz, so perhaps there is a quicker way to do this. Anyway, I think this is a very versatile technique, and that has some elegancy too!

Finally, I think this table is acquirable using normal TeX. Personally, I would prefer that for use in documents. Ask me if you need assistance on that.

Answer 2

With {NiceTabular} of nicematrix (version ≥ 6.23).

\documentclass{article}
\usepackage{nicematrix}
\begin{document}
$\begin{NiceArray}{c|c}[rounded-corners,hvlines,columns-width=20mm,color-inside]
  \rowcolor{cyan!45}
    x  & y = 2x+1 \
  \Hline
  \Block[fill=cyan!7]{-1}{} % color for the first column
    -3 & \Block[fill=cyan!7]{-1}{} % color for the second column
         -5 \
    -2 & -3 \
    -1 & -1 \
    0  & 1  \
    1  & 3  \
    2  & 5  \
    3  & 7  \
\end{NiceArray}$
\end{document}

You need several compilations (because nicematrix uses PGF/Tikz nodes under the hood).