Why is `\everypar` disturbed by a non-empty pair of brackets at the beginning of the following paragraph?

Question

At the end of a custom pseudo-list environment, I re-define \@doendpe to prevent indents in the paragraphs immediately following the environment, regardless on whether there is a blank line after the \end{env} (1st case) or not (2nd case). This works fine for most use cases (1st through 3rd cases).

However, if there is a non-empty pair of curly brackets at the beginning of the paragraph immediately following the environment, the no-indent spreads to the next paragraph as well (4th case). If the group is empty, it works as expected (5th case):

\documentclass{article}
\parindent8mm
\author{}
\title{}
\makeatletter
\let\orig@doendpe@doendpe
\newenvironment{mylist}
  {\list{}{%
      \leftmargin0pt
      \parindent0pt
      \bfseries\sffamily
    }\item\relax}
  {\endlist
   \gdef@doendpe{%
     @endpetrue
     \everypar{{\setbox\z@\lastbox}\everypar{}@endpefalse}%
     \global\let@doendpe\orig@doendpe}%
}
\makeatother
\begin{document}
\begin{mylist}
  1st case
\end{mylist}
test test test
test test test
test test test
\begin{mylist}
  2nd case
\end{mylist}
test test test
test test test
test test test
\begin{mylist}
  3rd case
\end{mylist}
[test test test]
test test test
test test test
\begin{mylist}
  4th case
\end{mylist}
{[}test test test{]}
test test test
test test test
\begin{mylist}
  5th case
\end{mylist}
{}test test test
test test test
test test test
\end{document}

Output:

I'd like to know (1), why this solution breaks when there is a non-empty pair of brackets immediately after the \end (and why it doesn't break when the brackets are empty) and (2), what would I need to change in the definition of the environment to make it work with all five use cases.

Answer 1

Your braces in case 4 with the text in it mean that the paragraph is started in a group. In this group \everypar is executed and it does reset itself but only locally, and so after the group the "doendpe-everypar" magic to reset itself is lost again.

You could use a para hook instead:

\documentclass{article}
\parindent8mm
\author{}
\title{}
\newenvironment{mylist}
  {\list{}{%
      \leftmargin0pt
      \parindent0pt
      \bfseries\sffamily
    }\item\relax}
  {\endlist\AddToHookNext{para/begin}{\OmitIndent}}
\begin{document}
\begin{mylist}
  1st case
\end{mylist}
test test test
test test test
test test test
\begin{mylist}
  2nd case
\end{mylist}
test test test
test test test
test test test
\begin{mylist}
  3rd case
\end{mylist}
[test test test]
test test test
test test test
\begin{mylist}
  4th case
\end{mylist}
{[}test test test{]}
test test test
test test test
\begin{mylist}
  5th case
\end{mylist}
{}test test test
test test test
test test test
\end{document}