Hello again,
About the Polyglossia bug: I still don't know exactly what's
happening, but I am now reasonably convinced that it happens within
Polyglossia's code (even though it's provoked by issuing fontspec
commands), and that defining \cyrillicfont will reliably work around the
problem -- see attachments.
To be continued for a proper bugfix.
Best,
Arthur
\documentclass{article}
\usepackage{polyglossia}
\setmainlanguage{russian}
\defaultfontfeatures{Scale=MatchLowercase}
\setmainfont{Linux Libertine O}
\begin{document}
Привет Мир!
\end{document}
\documentclass{article}
\usepackage{polyglossia}
\setmainlanguage{russian}
\defaultfontfeatures{Scale=MatchLowercase}
\setmainfont{Linux Libertine O}
\newfontfamily\cyrillicfont[Script=Cyrillic]{Linux Libertine O}
\begin{document}
Привет Мир!
\end{document}
