中英文混排时，等宽英文前后空格宽度不一致

[Evian-Zhang]

\documentclass{ctexart}
\begin{document}
你\texttt{abc}好。
\end{document}

生成的结果中，“你”之后“a“之前的空格为等宽字体的空格，宽度为1em；“c”之后“好”之前的空格为正常字体的空格，宽度为0.5em。

编译器：XeLaTeX 平台：macOS CTeX版本：2021/03/14 v2.5.6 详细信息：

xelatex --version
XeTeX 3.14159265-2.6-0.999992 (TeX Live 2020)
kpathsea version 6.3.2
Copyright 2020 SIL International, Jonathan Kew and Khaled Hosny.
There is NO warranty.  Redistribution of this software is
covered by the terms of both the XeTeX copyright and
the Lesser GNU General Public License.
For more information about these matters, see the file
named COPYING and the XeTeX source.
Primary author of XeTeX: Jonathan Kew.
Compiled with ICU version 65.1; using 65.1
Compiled with zlib version 1.2.11; using 1.2.11
Compiled with FreeType2 version 2.10.1; using 2.10.1
Compiled with Graphite2 version 1.3.13; using 1.3.13
Compiled with HarfBuzz version 2.6.4; using 2.6.4
Compiled with libpng version 1.6.37; using 1.6.37
Compiled with poppler version 0.68.0
Using Mac OS X Core Text and Cocoa frameworks

[sikouhjw]

https://github.com/CTeX-org/forum/issues/165

[muzimuzhi]

生成的结果中，“你”之后“a“之前的空格为等宽字体的空格，宽度为1em；“c”之后“好”之前的空格为正常字体的空格，宽度为0.5em。

宽度描述那里，不大对。

本地测试的结果，单位均为 pt

• 字号为 10.53937，对应 1em 的宽度
• 你 <\glue 5.53317> a
• c <\glue 3.50961 plus 1.7548 minus 1.16986> 好

\documentclass{ctexart}
\showboxbreadth=1000
\showboxdepth=100

\begin{document}
\setbox0=\hbox{你\texttt{abc\quad abc}好}
\showbox0
\usebox0
\end{document}

log 里得到

> \box0=
\hbox(8.06262+1.82329)x73.8599
.\TU/SongtiSCLight(0)/m/n/10.53937 你
.\kern 0.0
.\glue 5.53317
.\TU/lmtt/m/n/10.53937 abc
.\kern -0.0002
.\kern 0.0002
.\glue 10.53937
.\TU/lmtt/m/n/10.53937 abc
.\kern 0.0
.\glue 3.50961 plus 1.7548 minus 1.16986
.\TU/SongtiSCLight(0)/m/n/10.53937 好
.\kern -0.00017
.\kern 0.00017

! OK.
l.8 \showbox0

[stone-zeng]

See #252.