Add \par only if last paragraph did not end with displayed math

Question

Background:

I have named "sections" (sections for rest of this question) which are conditionally displayed or suppressed based on parameters. To simplify the test case below, these are controlled by the three \defs following \begin{document}.

Each of these sections should be able to control whether they are typeset in a new paragraph or are to continue from the last. This seems to be working for 3/4 of the cases. Where I run into a problem is if the last enabled section ended in a display math environment, and the following section wants to always be displayed as a new paragraph (that is not continue on as part of the previous paragraph as is the case for the IntroDetails paragraph.

Problem:

The MWE below reproduces the problem case, and you can see that the Summary paragraph starts further down then it should. To see this more clearly, you can see the other cases that work, which is any combination of commenting out at least one of the following:

\def\EndWithDisplayMath{}% 
\def\SupressIntroDetails{}%

So, it seems what I need is a macro that redefines \par if the last typeset content ended in display math. I would like to add the intelligence to the \NamedSection macro so that each of the named content sections don't need to take into consideration if the previous typeset content ended with displayed math.

Notes:

• In case it is relevant, the sections are always typset in the order they appear in the document, but each section can be typeset or suppressed independently of the others.
• I attempted to reproduce the problem within a minipage environment to make it easier to see the problem spacing, but things seems to work just fine in a minipage?

References:

• I am using a modified version of the solution from Remove excess space at end to eliminate the vertical space if the section ended with with a display math equation. The modification was to remove the \par which is most likely related to the \RemoveSpaceAtEnd not quite working.

Code:

\def\EndWithDisplayMath{}% Problem ONLY if BOTH of these are uncommented.
\def\SupressIntroDetails{}%
\documentclass{article}
\usepackage{showframe}
\usepackage{parskip}
\usepackage{xparse}
% https://tex.stackexchange.com/questions/45943/remove-excess-space-at-end
\newcommand{\RemoveSpaceAtEnd}[1]{%
    \begingroup
        \advance\belowdisplayskip1sp
        \advance\belowdisplayshortskip1sp
        %\par% Removed this from solution from 45943
        #1%
        %\par% Removed this from solution from 45943
        \ifdim\lastskip=\belowdisplayshortskip
            %\typeout{display here (short)}%
            \nobreak
            \vskip-\belowdisplayshortskip
        \else
            \ifdim\lastskip=\belowdisplayskip
                %\typeout{display here \noexpand#1}%
                \nobreak
                \vskip-\belowdisplayskip
            \fi
        \fi
    \endgroup
}
\NewDocumentCommand{\NamedSection}{%
    m% {#1} = name for sub-section
   +m% {#2} = content ("+" can have para)
}{%
    \ignorespaces%
    \ifcsname#1\endcsname%
        \RemoveSpaceAtEnd{#2}%
    \fi%
    \ignorespacesafterend%
}
\begin{document}
% Control which sections to display:
\def\Intro{}%
\ifdefined\SupressIntroDetails
\else
    \def\IntroDetails{}%
\fi
\def\Summary{}%
\NamedSection{Intro}{%
    First we introduce the topic.
    \ifdefined\EndWithDisplayMath
        [
            E = mc^2.
        ]
    \fi
}%
\NamedSection{IntroDetails}{%
Followed by more intro details.
}%
%
% Leaving a blank line here works fine, except if "IntroDetails" are supressed.
\NamedSection{Summary}{%
%  Want this to start on its own line.
\par%  Using \par works fine, except if "IntroDetails" are supressed.
And finally summarize the topic.
}%
\end{document}

Answer 1

As far as I can see you just want to put my \par back and then

\NewDocumentCommand{\NamedSection}{%
    m% {#1} = name for sub-section
   +m% {#2} = content ("+" can have para)
}{%
    \ignorespaces%
    \ifcsname#1\endcsname%
        \RemoveSpaceAtEnd{#2}%
    \fi%
    \ignorespaces%afterend%
}

---

If you need the sections sometimes to be mid-paragraph then you can restrict the use of \par to the beginning of a horizontal list, which means it's either after $$ or \noindent (or you are in an inner list where \par does nothing anyway) or you are doing something very sneaky.

\makeatletter

\advance\belowdisplayskip1sp
\advance\belowdisplayshortskip1sp
\newcommand{\RemoveSpaceAtEnd}[1]{%
        %\par% Removed this from solution from 45943
        #1%
        \def\next{}%
        \ifhmode\ifnum\lastnodetype=-1\par
          \let\next\@doendpe
        \fi% Removed this from solution from 45943
        \ifdim\lastskip=\belowdisplayshortskip
            %\typeout{display here (short)}%
            \nobreak
            \vskip-\belowdisplayshortskip
        \else
            \ifdim\lastskip=\belowdisplayskip
                %\typeout{display here \noexpand#1}%
                \nobreak
                \vskip-\belowdisplayskip
            \fi
        \fi
        \next
}