How to avoid typing \leavevmode before a citation macro that already starts with \leavevmode?

Question

Control sequences with optional arguments inside optional arguments must be "hidden" with braces. Using the abntex2cite package to handle my citations, I often want to write:

\begin{teor}[{\cite[p.~42]{johndoe1999}}]
  $2+2=4$.
\end{teor}

That doesn't work, however; I need to write:

\begin{teor}[\leavevmode{\cite[p.~42]{johndoe1999}}]
  $2+2=4$.
\end{teor}

instead. \cite and \citeonline behave exactly the same in this regard. Besides \leavevmode, a \strut or just a space/end-of-line also work (except the space looks weird/ugly). That is, LaTeX needs something to start a line right before the citation macro between braces. I thought that maybe this happens because the macros \cite and \citeonline had to start with \leavevmode, but don't; however, the source of abntex2cite shows that the very first line of \citeonline IS \leavevmode!

So my questions are:

• Why is another \leavevmode before \citeonline (or \cite) necessary, even though \citeonline starts with \leavevmode?
• Is there a way to redefine \cite and \citeonline such that I don't need space/\strut/\leavevmode before the braced citation macro?

(Files to showcase what I described)

Contents of sample.tex:

\documentclass{article}
\usepackage{ntheorem}
\newtheorem{teor}{Teorema}
\newcommand*{\page}[1]{p.~#1}
\usepackage[alf]{abntex2cite}


\begin{document}
Regular ``external'' citation according to the ABNT (Brazilian) standard:
\cite{johndoe1999}.

Regular ``inline'' citation according to the ABNT (Brazilian) standard:
\citeonline{johndoe1999}.

So far, so good; no extra braces needed.
Whithin the optional argument of an environment, however,
extra braces are needed:

\begin{teor}[This works
  {\cite[\page{25}]{johndoe1999}}%<-- notice the extra braces!
  ]
  Notice the extra braces in the optional argument.
\end{teor}

\begin{teor}[Also works
  {\citeonline[\page{25}]{johndoe1999}}%<-- notice the extra braces!
  ]
  Notice the extra braces in the optional argument.
\end{teor}

And the next one works, but it requires
(besides the extra braces)
something to start a line before \verb|\cite| or \verb|\citeonline|:
it can be a space, a \verb|\strut| or \verb|\leavevmode|.
I think that's annoying and error-prone;
is there another way?

\begin{teor}[% avoid extraneous space here
  \leavevmode{\citeonline[\page{25}]{johndoe1999}}%<-- \leavevmode + extra braces!
  ]
  This one requires both
  extra braces AND ``something to start a line''
  in the optional argument.
\end{teor}

\bibliography{bibfile}
% this is how a chapter of a book is supposed to look in the ABNT style...
\end{document}

% -- the following won't compile!
% erase the previous 2 macros to generate errors
\begin{teor}[% lacks \leavevmode AND extra braces
  \citeonline[\page{25}]{johndoe1999}%
  ]
  \fbox{content}
\end{teor}

\begin{teor}[% lacks extra braces: {\citeonline...}
  \leavevmode\citeonline[\page{25}]{johndoe1999}%
  ]
  \fbox{content}
\end{teor}

\begin{teor}[% lacks \leavevmode (or \strut)
  {\citeonline[\page{25}]{johndoe1999}}%
  ]
  \fbox{content}
\end{teor}

\bibliography{bibfile}
\end{document}

Contents of bibfile.bib:

% Encoding: UTF-8
@InCollection{johndoe1999,
  author    = {Doe, John},
  booktitle = {Handbook of Foobar},
  publisher = {Barbaz},
  title     = {Interesting chapter},
  year      = {1999},
  address   = {Earth},
  editor    = {{Mc}Editor, Editor},
  pages     = {283--342},
}

Answer 1

The problem is obviously due to ntheorem: if I remove it, the issue disappears.

If you really need it (personally, I wouldn't use it), I can propose a workaround:

\begin{filecontents*}{\jobname.bib}
@InCollection{johndoe1999,
  author    = {Doe, John},
  booktitle = {Handbook of Foobar},
  publisher = {Barbaz},
  title     = {Interesting chapter},
  year      = {1999},
  address   = {Earth},
  editor    = {{Mc}Editor, Editor},
  pages     = {283--342},
}
\end{filecontents*}

\documentclass{article}
\usepackage[alf]{abntex2cite}
\usepackage{ntheorem}

\newtheorem{teor}{Teorema}

\ExplSyntaxOn
\NewDocumentCommand{\fixtheorem}{m}
 {
  \cs_set_eq:cc { #1@fixed } { #1 }
  \cs_set_eq:cc { end#1@fixed } { end#1 }
  \RenewDocumentEnvironment{#1}{o}
   {
    \IfNoValueTF{##1}{\use:c{#1@fixed}}{\use:c{#1@fixed}[\exp_not:n{##1}]}
   }
   {
    \use:c{end#1@fixed}
   }
 }
\ExplSyntaxOff

\fixtheorem{teor}

\newcommand*{\page}[1]{p.~#1}

\begin{document}

Regular ``external'' citation according to the ABNT (Brazilian) standard:
\cite{johndoe1999}.

Regular ``inline'' citation according to the ABNT (Brazilian) standard:
\citeonline{johndoe1999}.

So far, so good; no extra braces needed.
Whithin the optional argument of an environment, however,
extra braces are needed:

\begin{teor}[This works {\cite[\page{25}]{johndoe1999}}]
Notice the extra braces in the optional argument.
\end{teor}

\begin{teor}[Also works {\citeonline[\page{25}]{johndoe1999}}]
Notice the extra braces in the optional argument.
\end{teor}

And the next one works, but it requires
(besides the extra braces)
something to start a line before \verb|\cite| or \verb|\citeonline|:
it can be a space, a \verb|\strut| or \verb|\leavevmode|.
I think that's annoying and error-prone;
is there another way?

\begin{teor}[{\cite[\page{25}]{johndoe1999}}]
This one requires both extra braces AND ``something to start a line'' 
in the optional argument.
\end{teor}

\begin{teor}[{\citeonline[\page{25}]{johndoe1999}}]
This one requires both extra braces AND ``something to start a line'' 
in the optional argument.
\end{teor}

\bibliography{\jobname}

\end{document}

The original teor is renamed teor@fixed and a more robust teor environment is defined, with the optional argument protected from the untimely expansion done by ntheorem.

This also has a little advantage, in that you can even omit the inner braces and

\begin{teor}[\cite[\page{25}]{johndoe1999}]

would work.