Missing Qu-ligature with Libertinus + LuaLaTeX, but OK for Libertinus + XeLaTeX

Question

Compile this MWE with LuaLaTeX and with XeLaTex

\documentclass{article}
\usepackage[defaultfeatures={Kerning=On,Ligatures={TeX,Common,Rare}}]{libertinus}
\begin{document}
\begin{tabular}{ccccccccccccc}
ff & fi & fj & fl & ft & ffi & ffj & ffl & fft & Th & ts & tt & Qu
\end{tabular}
\end{document}

The result for XeLaTeX (note the Qu-ligature):

The result for LuaLaTeX (note the missing Qu-ligature):

Expected result: The Qu-ligature should be there. What am I doing wrong? Is this a bug and if yes, is there a workaround until it is fixed upstream?

Answer 1

The problem is that you explicitly enabled "Ligatures", but Qu not implemented as a ligature here, "Q" has a "contextual alternate" (Another version of the "Q" is inserted depending on the context). If you enable these Contextual Alternates, Qu works:

\documentclass{article}
\usepackage[defaultfeatures={Script=Latin,Kerning=On,Ligatures={TeX,Common,Rare},Contextuals = Alternate}]{libertinus}
\begin{document}
\begin{tabular}{ccccccccccccc}
ff & fi & fj & fl & ft & ffi & ffj & ffl & fft & Th & ts & tt & Qu
\end{tabular}
\end{document}

In addition to enableing thislonger form of "Q" if followed by a "u" or "v", enabling Contextuals = Alternate has one other effect: If a f (or ff ligature) is followed by a relativly high character but does not form a ligature with this character, the "f" is replaced by a shorter form.

For example this shows up for fi if ligatures are disabled: The following shows first fi with common ligatures enabled, then fi with neither common ligatures nor contextual alternates and then fi without ligatures but with the alternate.