未编号的多列 tcolorbox 列表的对齐

Question

考虑以下代码：

% Preamble
\documentclass[letterpaper, 10pt]{article}
\usepackage{calc}
\usepackage{xcolor}
\usepackage{listings}
\usepackage{multicol}
\usepackage[most]{tcolorbox}
\usepackage{lipsum}

% Lengths
\newlength{\xshift}
\newlength{\tcbboxrule}
\newlength{\tcbleft}
\newlength{\tcbright}
\newlength{\lstxleftmargin}
\newlength{\lstxrightmargin}

% Parameters (that can be changed by the user)
\setlength{\xshift}{1em}
\setlength{\tcbboxrule}{1mm}

% Variables (to be adjusted automatically from the previous ones)
\setlength{\tcbleft}{0pt} % <- TO BE MODIFIED
\setlength{\tcbright}{0pt} % <- TO BE MODIFIED
\setlength{\lstxleftmargin}{\xshift} % <- TO BE MODIFIED
\setlength{\lstxrightmargin}{0pt} % <- TO BE MODIFIED

% Other options
\definecolor{backgroundcolor}{RGB}{240, 240, 240}
\definecolor{framecolor}{RGB}{24, 24, 128}
\colorlet{keywordcolor}{blue}
\colorlet{commentcolor}{red}
\newlength{\tcbboxrulemask}
\setlength{\tcbboxrulemask}{\dimexpr 0.5\tcbboxrule + 0.5pt \relax}

% Environments
\newtcblisting{multilisting}[3][\normalsize]{
    before upper = {\topskip0pt \maxdepth=0pt},
    enhanced,
    listing only,
    breakable = unlimited,
    top = 0em,
    bottom = 0em,
    arc = 0em,
    outer arc = 0em,
    boxsep = 0em,
    titlerule = 0em,
    colback = backgroundcolor,
    colframe = framecolor,
    boxrule = \tcbboxrule,
    left = \tcbleft,
    right = \tcbright,
    listing options = {
        language = C++,
        showstringspaces = false,
        literate = {~}{$\sim$}{1},
        framesep = 0pt,
        aboveskip = 0pt,
        belowskip = 0pt,
        xleftmargin = \lstxleftmargin,
        xrightmargin = \lstxrightmargin,
        basicstyle = \fontfamily{cmtt}\selectfont#1,
        keywordstyle = \color{keywordcolor},
        commentstyle = \color{commentcolor},
        tabsize = 4,
        numbers = none,
        multicols = 2,
        #2,
    },
    overlay = {
        \draw[line width = \tcbboxrule, framecolor]
            (interior.north) -- (interior.south);
        \fill[backgroundcolor] (interior.north) rectangle
            ([xshift = \tcbboxrulemask] interior.south);
    },
    #3,
}

% Document
\begin{document}
\lipsum[1]
\begin{multilisting}[\normalsize]{}{}
double f(double x)
{
    return 2 * x;
}

double g(double x)
{
    return 3 * x;
}
\end{multilisting}
\lipsum[1]
\end{document}

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它产生以下输出：

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以下是一些测量结果：

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在上面的代码中，fontsize列表的可以调整，以及\xshift最终\tcbboxrule用户可以调整。

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问题：如何更改标记的行，TO BE MODIFIED以便多列列表正确“居中” tcolorbox（意味着图上标记为的距离?x?等于\xshift）？