I would like to add a border to the left of some indented text, eventually spanning **several pages and nested**. My goal is similar to [Indentation level inside proof](https://tex.stackexchange.com/questions/334046/indentation-level-inside-proof) but the solution I found back that time was not working on text spanning several pages. For what I tried, `mdframed` and `tcolorbox` have issues with nested boxes spanning several pages (for example the inner box starts on a new page).

Meanwhile, I found the command `adjustwith` that let me indent my text in a pretty simple and robust way. But I still don't know how to add a vertical line to the left of the text.

Here is what I want:

[![enter image description here](https://i.stack.imgur.com/LjagE.png)](https://i.stack.imgur.com/LjagE.png)

And what I have so far:

[![enter image description here](https://i.stack.imgur.com/fEXSs.png)](https://i.stack.imgur.com/fEXSs.png)

MWE:

```
\documentclass{article}
\usepackage{changepage}% http://ctan.org/pkg/changepage
\usepackage{lipsum}% http://ctan.org/pkg/lipsum

\newenvironment{subproof}{\begin{adjustwidth}{2.5em}{0pt}}{\end{adjustwidth}}

\begin{document}
\section{Title}
I would like to add a black border on the left of all ``adjustwidth'' blocks:
\begin{subproof}
  \lipsum[1]
  \begin{subproof}
    \lipsum[1-2]
    \[\Delta = b^2 - 4ac\]
    \lipsum[1-2]
  \end{subproof}
  \lipsum[1]
\end{subproof}
\lipsum[1]
\end{document}

```

EDIT: I just heard of the `changebar` package, and it does something quite close to the thing I want... except that I can't find how to change the position of the line several times. Indeed, if I change `\changebarsep` after the beginning of the document, the lines position are not changed... I was thinking to modify the package to my need, but I'm not sure to see yet how to do...

EDIT2:

The proposition of muzimuzhi does not work, because when the nested subproof is larger than a page, it it moved to the next page (leaving a bit blank in the first page), and the bars are a bit ugly as on the next page the inner bars are longer than the outer bars:

[![enter image description here](https://i.stack.imgur.com/BRfCh.png)](https://i.stack.imgur.com/BRfCh.png)

Code to produce this result:

```
\documentclass{article}
\usepackage{lipsum}
\usepackage{tcolorbox}
\tcbuselibrary{breakable, skins}

\newtcolorbox{lmarginbox}{
  blanker, breakable, left=2.5em,
  borderline west={1pt}{0pt}{black}}

\begin{document}
\lipsum[33]

\begin{lmarginbox}
  \lipsum[1-2]
  \begin{lmarginbox}
    \lipsum[1-5]
  \end{lmarginbox}
  \lipsum[4-5]
\end{lmarginbox}

\lipsum[33]
\end{document}

```

-- EDIT 3 --

I found also [this answer](https://tex.stackexchange.com/questions/367048/drawing-tikz-line-in-the-margin-for-multiple-pages) which seems to provide some tricks to draw lines between several pages using tikz. I guess it could be a solution...