Good morning!

This report by Github user SverreStausland has been lingering on
the Luaotfload tracker for a while: There appears to be a
difference between how certain faces of the Libertine family
handle the placement of diacritics.

With Version 5.1.2 of the family [0], the "RZI" version
(fullname: “Linux Libertine O Semibold Italic”) shows a different
behavior than its “RZ” companion (“Linux Libertine O Semibold”).
Short demonstration in Context:

    \definefontfeature [ourfeats] [default] [mode=node]

    \starttext
      \start \definedfont [file:LinLibertine_RI.otf*ourfeats]  V\char"0306 \stop
      \start \definedfont [file:LinLibertine_RZI.otf*ourfeats] V\char"0306 \stop
    \stoptext

Note the misaligned combining breve above the latter. An
equivalent test for Plain [1] shows that the correct position
information is there somewhere in the font, since the Xetex
engine typesets it correctly.

Best regards,
Philipp

[0] MD5 hashes of the files:

    4bfd3481c1486f5d1ea13304981df7a6  LinLibertine_RZI.otf
    153765bd2b29d6026fb2fa0b198a4995  LinLibertine_RZ.otf

[1] https://bitbucket.org/phg/lua-la-tex-tests/src/tip/pln-diacritics-1.tex