[luatex] strange behavior of zero-width box

[Dohyun Kim]

Hi,

\font\X={Junicode-Regular:mode=node;salt=1}
\font\Y={Junicode-Regular:mode=node;salt=2}
\leavevmode\X\hbox{1\llap{\Y2}}3
\bye

On compiling the above code with luatex-plain,
I get a pdf in which "2" is overwritten on "3".  This is not correct, of course.

Now if I insert "\kern-52sp" (why 52sp? I don't know.)
between "1" and "\llap", that is

\leavevmode\X\hbox{1\kern-52sp\llap{\Y2}}3

then "2" is correctly overwitten over "1".

I am not sure whether this problem comes from luatex engine
or from luatex-plain macro.  So I'm posting here.

Best,
-- 
Dohyun Kim
College of Law, Dongguk University
Seoul, Republic of Korea