The same as plain tex to cut down the dependencies:-)
% https://tex.stackexchange.com/questions/401234/how-to-get-a-mid-binary-relation-that-grows-in-luatex \tracingonline1 \showboxbreadth100 \scrollmode \thickmuskip=10mu $\left(a\mathrel{}\middle|\mathrel{}b\right)\showlists$\showlists \bye the math lists are similar in pdftex and luatex but the box output has an extra empty box and an extra \thickuskip skip David
