[luatex] \vsplit issue

[Joseph Wright]

Hello all,

The example

    \setbox0=\vbox{a\par b\par c\par d}
    \setbox2=\vsplit0 to 2cm %
    %\showboxbreadth\maxdimen
    %\showboxdepth\maxdimen
    %\showbox2 %
    \bye

gives an underful vbox with pdfTeX, leading to a warning and box trace
in the log. The same isn't the case for LuaTeX (v0.95 from TL'16), which
comes from the fact the box is no longer the same size:

    > \box2=
    \vbox(40.30554+0.0)x469.75499, direction TLT

from LuaTeX, wherease pdfTeX gives

    > \box2=
    \vbox(56.9055+0.0)x469.75499, glue set 5.53333

An error?

Joseph