The number of repetition *for each* extender part was limited to 128 in bug 155434. If we support an arbitrary number of parts (thus extender parts) then the limit should instead be set on the number of glyphs *for the whole assembly*. This blog post explains how to implement generic support for the GlyphAssembly sub-table and in particular to calculate (before painting) the number N of glyphs necessary (and so to set a limit N_max): http://frederic-wang.fr/opentype-math-in-harfbuzz.html When the limit is exceeded, we currently just leave a big gap. In bug 155434 comment 6 it was suggested to evenly distribute the gaps. I would instead suggest to just limit the number N of glyphs (and as a consequence to limit the stretch size) before painting them. The blog post gives a reference to an implementation in progress for HarfBuzz. I think it makes sense to use the text drawing (MathOperator class) as well as MATH table parsing (OpenTypeMathData class) from such library. In particular for WebKit ports using HarfBuzz (such as GTK) we should probably consider reusing HarfBuzz's MATH support when ready.