20 如何自建相关命令来取代 `tcolorbox` 的 `tcblower` 这个wheel？

封装一下 minipage，设置 \parindent=0pt。

\documentclass{article}

\ExplSyntaxOn
\box_new:N \l__skyrmion_left_box
\box_new:N \l__skyrmion_right_box
\box_new:N \l__skyrmion_seg_box
\keys_define:nn { skyrmion }
  {
    lefthand~ratio .fp_set:N = \l__skyrmion_left_ratio_fp ,
    lefthand~width .dim_set:N = \l__skyrmion_left_dim ,
    righthand~ratio .dim_set:N = \l__skyrmion_right_ratio_fp ,
    righthand~width .dim_set:N = \l__skyrmion_right_dim ,
    sidebyside~gap .dim_set:N = \l__skyrmion_gap_dim ,
    sidebyside~gap .initial:n = 10mm ,
    before~left .tl_set:N = \l__skyrmion_before_left_tl ,
    before~right .tl_set:N = \l__skyrmion_before_right_tl ,
  }
\cs_new_protected:Npn \twopartsplit
  {
    \dim_compare:nNnTF \l__skyrmion_right_dim < { 10sp }
      {
        %% Width is too small! Abort!
      }
      {
        \__skyrmion_twopart_end_save:
        \__skyrmion_twopart_save:NN \l__skyrmion_right_box \l__skyrmion_right_dim
        \l__skyrmion_before_right_tl \ignorespaces
      }
  }
\NewDocumentEnvironment{twopart}{ O{} }
  {
    \par \keys_set:nn { skyrmion } {#1}
    \__skyrmion_calc_width: 
    \__skyrmion_twopart_save:NN \l__skyrmion_left_box \l__skyrmion_left_dim
    \l__skyrmion_before_left_tl \ignorespaces
  }
  {
    \__skyrmion_twopart_end_save:
    \__skyrmion_twopart_typeset:
  }
\cs_new:Npn \__skyrmion_calc_width:
  {
    \bool_lazy_and:nnTF
      { \dim_compare_p:nNn \l__skyrmion_left_dim = \c_zero_dim }
      { \dim_compare_p:nNn \l__skyrmion_right_dim = \c_zero_dim }
      {
        \bool_lazy_and:nnTF
          { \fp_compare_p:nNn { abs ( \l__skyrmion_left_ratio_fp ) } < { 0.0001 } }
          { \fp_compare_p:nNn { abs ( \l__skyrmion_right_ratio_fp ) } < { 0.0001 } }
          {
            \dim_set:Nn \l__skyrmion_left_dim { 0.5\linewidth - 0.5\l__skyrmion_gap_dim }
            \dim_set:Nn \l__skyrmion_right_dim { \l__skyrmion_left_dim }
          }
          {
            \fp_compare:nNnTF { abs ( \l__skyrmion_left_ratio_fp ) } < { 0.0001 }
              {
                \dim_set:Nn \l__skyrmion_right_dim 
                  { 
                    \fp_use:N \l__skyrmion_right_ratio_fp 
                    \dimexpr \linewidth - \l__skyrmion_gap_dim \relax
                  }
                \dim_set:Nn \l__skyrmion_left_dim 
                  { \linewidth - \l__skyrmion_gap_dim - \l__skyrmion_right_dim }
              }
              {
                \dim_set:Nn \l__skyrmion_left_dim 
                  { 
                    \fp_use:N \l__skyrmion_left_ratio_fp 
                    \dimexpr \linewidth - \l__skyrmion_gap_dim \relax
                  }
                \dim_set:Nn \l__skyrmion_right_dim 
                  { \linewidth - \l__skyrmion_gap_dim - \l__skyrmion_left_dim }
              }
          }
      }
      {
        \dim_compare:nNnTF \l__skyrmion_left_dim = \c_zero_dim
          {
            \dim_set:Nn \l__skyrmion_left_dim 
              { \linewidth - \l__skyrmion_gap_dim - \l__skyrmion_right_dim }
          }
          {
            \dim_set:Nn \l__skyrmion_right_dim 
              { \linewidth - \l__skyrmion_gap_dim - \l__skyrmion_left_dim }
          }
      }
  }
\cs_new:Npn \__skyrmion_twopart_save:NN #1 #2
  {
    \hbox_set:Nw #1
    \minipage {#2}
    \setlength{\parindent}{0pt}
  }
\cs_new:Npn \__skyrmion_twopart_end_save:
  {
    \endminipage
    \hbox_set_end:
  }
\cs_new:Npn \__skyrmion_seg: { } % 画线，可以用 tikz 等等
\cs_new:Npn \__skyrmion_twopart_typeset:
  {
    \hbox_to_wd:nn { \linewidth }
      {
        \hbox_set:Nn \l__skyrmion_seg_box { \__skyrmion_seg: }
        \hbox_unpack_drop:N \l__skyrmion_left_box
        \hss \clap { \box_use_drop:N \l__skyrmion_seg_box } \hss
        \hbox_unpack_drop:N \l__skyrmion_right_box
      }
  }

\ExplSyntaxOff

\usepackage{lipsum,amsthm,tcolorbox}
\NewTColorBox { textfig } { O { .64\linewidth } }
  {
    sidebyside, sidebyside gap = .02\linewidth, boxsep = 0pt,
    left = 0pt, right = 0pt, top = 3pt, bottom = 0pt,
    lefthand width = #1, standard jigsaw, opacityback = 0, frame empty
  }
\newtheorem{problem}{Problem}

\begin{document}

\begin{textfig}
  \begin{problem}
    \lipsum[2]
  \end{problem}
  \tcblower
  \centering
  \includegraphics[width = .5\linewidth]{example-image-a}
\end{textfig}

\begin{twopart}[sidebyside gap=.02\linewidth, lefthand width=.64\linewidth]
  \begin{problem}
    \lipsum[2]
  \end{problem}
\twopartsplit
  \centering
  \includegraphics[width = .5\linewidth]{example-image-a}
\end{twopart}

\end{document}

也可以用 coffin，代码是相似的。