microtype/ntheorem: Protrusion not affecting headers of the theorems

Question

I'm trying to switch to the ntheorem package (instead of amsthm) but I realized that headers of the theorems aren't protruded:

\documentclass{article}
\usepackage[activate={true, nocompatibility}, factor=2000]{microtype}
\usepackage{ntheorem}
\usepackage[showframe]{geometry}

\makeatletter
\newtheoremstyle{mystyle}
{\item[\hskip\labelsep \theorem@headerfont ##1\ ##2\theorem@separator]}
{\item[\hskip\labelsep \theorem@headerfont ##1\ ##2\ (##3)\theorem@separator]}
\makeatother

\theoremstyle{mystyle}
\theorembodyfont{\upshape}
\theoremseparator{.}
\newtheorem{what}{Whatever}

\begin{document}
    \noindent\bfseries Whatever.\normalfont

    \begin{what}
        Something here.\newline
        Whatever.
    \end{what}
\end{document}

I've tried this and \theorem@headerfont\protrudeleft{##1} makes protrusion correct - but also it generates a few errors and affects formatting of the theorem. So how should I fix it? Thank you in advance for help.

Answer 1

The problem with the code you tried from the other answer is that here the unboxing takes place inside another (incompatible) box, which makes tex stumble. The solution is, just as you suggested in your comment, to measure the protrusion first and then subtract it from the label sep:

\documentclass{article}
\usepackage[activate={true, nocompatibility}, factor=2000]{microtype}
\usepackage{ntheorem}
\usepackage[showframe]{geometry}

\makeatletter
\makeatletter
\newcommand*\protrudeleft[1]{%
    {\everypar{}%
     \setbox\z@\vbox{\noindent#1}%
     \vbadness=\@M
     \splittopskip=\z@
     \global\setbox\z@=\vsplit\z@ to \baselineskip
     \unvbox\z@ \global\setbox\z@=\lastbox
    }%
    \@tempdima=\leftmarginkern\z@ 
}
\newtheoremstyle{mystyle}
{\protrudeleft{\theorem@headerfont ##1}%% measure left margin kern
 \item[\hskip\dimexpr\labelsep+\@tempdima %% add to labelsep
 \theorem@headerfont ##1\ ##2\theorem@separator]}
{\protrudeleft{\theorem@headerfont ##1}%
 \item[\hskip\dimexpr\labelsep+\@tempdima 
 \theorem@headerfont ##1\ ##2\ (##3)\theorem@separator]}
\makeatother

\theoremstyle{mystyle}
\theorembodyfont{\upshape}
\theoremseparator{.}
\newtheorem{what}{Whatever}

\begin{document}
    \noindent\bfseries Whatever.\normalfont

    \begin{what}
        Something here.\newline
        Whatever.
    \end{what}
\end{document}