\prebreakpenalty`…=0
\postbreakpenalty`「=0
\postbreakpenalty`【=0
\postbreakpenalty`（=0
\postbreakpenalty`：=0
\postbreakpenalty`；=0

\tracingonline1
\showboxdepth10000
\showboxbreadth10000

\font\x=jis\x
\setbox0=\hbox{あ…あ「あ【あ（あ：あ；あ}\showbox0
\box0

\font\y=skipjfmp at 9.62216pt\y
\setbox2=\hbox{あ…あ「あ【あ（あ：あ；あ}\showbox2
\box2

\newfam\mx
\newfam\my
\textfont\mx=\x \scriptfont\mx=\x \scriptscriptfont\mx=\x
\textfont\my=\y \scriptfont\my=\y \scriptscriptfont\my=\y

\jfam\mx
\setbox4=\hbox{$あ…あ「あ【あ（あ：あ；あ$}\showbox4
\box4

\jfam\my
\setbox6=\hbox{$あ…あ「あ【あ（あ：あ；あ$}\showbox6
\box6

\bye
