Math fills entire column with multicol and enumitem

Question

With the following code:

\documentclass[12pt]{article}

\usepackage[margin=2.7cm]{geometry}
\usepackage{multicol,enumitem}

\begin{document}
    
\begin{enumerate}[itemindent=\parindent]
    \setlength\columnsep{4em}
    \begin{multicols}{3}
        \everymath{\displaystyle}
        \item $\int_0^5 \big(x^5 - \exp(-x)\big)dx$.\hfill
        \item $\int_0^\pi x\sin(x)dx$.
        \item $\int_0^{\log 3}x\sinh(2x)dx$.
        \item $\int_{-1}^1 x^2\exp(-x)dx$.
        \item $\int_{-1}^1 x^3\exp(-x)dx$.
        \item $\int_{-19^{75}}^{19^{75}} x^3dx$.
        \item $\int_{-1}^2 x^3dx$.
        \item $\int_0^2 x^4dx$.
        \item $\int_{-2}^2x^4dx$.
        \item $\int_{-5}^{-1} (x+3)^3dx$.
        \item $\int_{-\pi}^{\pi} x^3\cos(x^2)dx$.
        \item $\int_{-\log 7}^{\log 7}\sinh(2x)dx$.
        \item $\int_{-\log 7}^{\log 7}\cosh(2x)dx$.
        \item $\int_0^{+\infty} x^2\exp(-x)dx$.
        \item $\int_{-\infty}^{+\infty} x^2\exp(-x^2)dx$.
    \end{multicols}
\end{enumerate}
    
\end{document}

I get the output of the first picture; as you can see with the first item, if the expression is too long, it tends to fill the entire space of the column. However, if I comment out the itemident=\parindent option (with the rest of the code unchanged), I get the second picture, which looks as one would expect this sort of thing to look. How do I solve this? I really need the itemindent parameter here, because I encountered this problem when using a customised list declared with enumitem's \newlist and \setlist. Thanks!

EDIT: The second picture is also produced if I comment out the line with the \columnsep macro.

Answer 1

I would (a) exchange the order of the itemize and multicols environments and (b) replace \begin{enumerate}[itemindent=\parindent] with \begin{enumerate}[left=0pt]. Semi-optionally, insert thinspace before all instances of dx.

\documentclass[12pt]{article}
\usepackage[margin=2.7cm]{geometry}
\usepackage{multicol,enumitem}
\setlength\columnsep{4em}

\begin{document}
\begin{multicols}{3}
\begin{enumerate}[left=0pt]   
\everymath{\displaystyle}
   \item $\int_0^5 \big(x^5 - \exp(-x)\big)\,dx$
   \item $\int_0^\pi x\sin(x)\,dx$
   \item $\int_0^{\log 3}x\sinh(2x)\,dx$
   \item $\int_{-1}^1 x^2\exp(-x)\,dx$
   \item $\int_{-1}^1 x^3\exp(-x)\,dx$
   \item $\int_{-19^{75}}^{19^{75}} x^3\,dx$
   \item $\int_{-1}^2 x^3\,dx$
   \item $\int_0^2 x^4\,dx$
   \item $\int_{-2}^2 x^4\,dx$
   \item $\int_{-5}^{-1} (x+3)^3\,dx$
   \item $\int_{-\pi}^{\pi} x^3\cos(x^2)\,dx$
   \item $\int_{-\log 7}^{\log 7}\sinh(2x)\,dx$
   \item $\int_{-\log 7}^{\log 7}\cosh(2x)\,dx$
   \item $\int_0^{+\infty} x^2\exp(-x)\,dx$
   \item $\int_{-\infty}^{+\infty} x^2\exp(-x^2)\,dx$
\end{enumerate}
\end{multicols}
\end{document}

Answer 2

It is simpler to do that with tasks package, which works like enumitem. You can force a task to spread over several columns with the optional argument \task(n).

\documentclass[12pt]{article}

\usepackage[margin=2.7cm]{geometry}
\usepackage{tasks}

\begin{document}

        \everymath{\displaystyle}\settasks{label=\arabic*., label-align=right, label-width=1.5em, ref=\arabic*}
    \begin{tasks}(3)
        \task $\int_0^5 \big(x^5 - \exp(-x)\big)dx$
        \task $\int_0^\pi x\sin(x)dx$.
        \task $\int_0^{\log 3}x\sinh(2x)dx$.
        \task $\int_{-1}^1 x^2\exp(-x)dx$.
        \task $\int_{-1}^1 x^3\exp(-x)dx$.
        \task $\int_{-19^{75}}^{19^{75}} x^3dx$.
        \task $\int_{-1}^2 x^3dx$.
        \task $\int_0^2 x^4dx$.
        \task $\int_{-2}^2x^4dx$.
        \task $\int_{-5}^{-1} (x+3)^3dx$.
        \task $\int_{-\pi}^{\pi} x^3\cos(x^2)dx$.
        \task $\int_{-\log 7}^{\log 7}\sinh(2x)dx$.
        \task $\int_{-\log 7}^{\log 7}\cosh(2x)dx$.
        \task $\int_0^{+\infty} x^2\exp(-x)dx$.
        \task $\int_{-\infty}^{+\infty} x^2\exp(-x^2)dx$.
    \end{tasks}

\end{document}