Anyone who thinks UNIX is intuitive should be forced to write 5000 lines of
code using nothing but vi or emacs. AAAAACK!
(Discussion in comp.os.linux.misc on the intuitiveness of commands, especially
Emacs.)

[paulo@cambridge ~]

\documentclass{article}

\showboxdepth100
\showboxbreadth100
\begin{document}

 hi $$x$$ ho \showlists,
\end{document}

### horizontal mode entered at line 7
\OT1/cmr/m/n/10 h
\OT1/cmr/m/n/10 o
\glue 3.33333 plus 1.66666 minus 1.11111
spacefactor 1000
### vertical mode entered at line 0
### current page:
\write-{}
\glue(\topskip) 3.05556
\hbox(6.94444+0.0)x345.0, glue set 321.66664fil
.\hbox(0.0+0.0)x15.0
.\OT1/cmr/m/n/10 h
.\OT1/cmr/m/n/10 i
.\penalty 10000
.\glue(\parfillskip) 0.0 plus 1.0fil
.\glue(\rightskip) 0.0
\penalty 10000
\glue(\abovedisplayshortskip) 0.0 plus 3.0
\glue(\baselineskip) 7.69446
\hbox(4.30554+0.0)x5.71527, shifted 169.64236, display

This is pdfTeX, Version 3.14159265-2.6-1.40.18 (TeX Live 2017) (preloaded format=pdflatex 2018.1.26)  8 FEB 2018 10:46

### horizontal mode entered at line 104
\localpar
.\localinterlinepenalty=0
.\localbrokenpenalty=0
.\localleftbox=null
.\localrightbox=null
\glue(\spaceskip) 2.20651 plus 1.10326 minus 0.7355
\TU/STIXTwoText(0)/m/n/9.3894 T

of the form
$$\partial_tu-A\varphi(u)=0.$$
These equations

$$\partial_tu-A\varphi(u)=0.$$ {}
These equations

\documentclass{article}

\RequirePackage{fontspec,unicode-math}
\setmainfont{STIX Two Text}
\setmathfont{STIX Two Math}

\begin{document}
\small
We study \ldots
\tracingcommands=2 \tracingmacros=2
$$\partial_tu-A\varphi(u)=0.$$
These equations \newline bla bla
\end{document}

\documentclass{article}


\begin{document}

We study \ldots
\tracingcommands=2 \tracingmacros=2
$$\partial_tu-A\varphi(u)=0.\aftergroup\begingroup\aftergroup\endgroup$$
These equations \newline bla bla
\end{document}