Non-stretching math accents

[davidmjones]

[This will be considered for v2.20 if there is time.]

There are a handful of accents that unicode-math expects to be stretchy that either don't stretch at all, or have limited stretchability.

Code	LaTeX Name	Unicode name
U+0302	widehat	COMBINING CIRCUMFLEX ACCENT
U+0303	widetilde	COMBINING TILDE
U+0306	widebreve	COMBINING BREVE
U+030C	widecheck	COMBINING CARON
U+0330	wideutilde	COMBINING TILDE BELOW
U+034D	underleftarrow	COMBINING LEFT RIGHT ARROW BELOW
U+20D0	overleftharpoon	COMBINING LEFT HARPOON ABOVE
U+20D1	overrightharpoon	COMBINING RIGHT HARPOON ABOVE
U+20EC	underrightharpoondown	COMBINING RIGHTWARDS HARPOON WITH BARB DOWNWARDS
U+20ED	underleftharpoondown	COMBINING LEFTWARDS HARPOON WITH BARB DOWNWARDS

Of course, in some cases the shape of the accent might make it difficult to extend it indefinitely.

Attached are examples generated using XeTeX. The results are the same in luaTeX.
accents-stix2math.pdf

[tiroj]

The circumflex, tilde and caron combining marks all max-out at s5 variants. We could add additional width variants, but I am unsure how wide is really necessary in practice.

The linear marks like the arrows, etc. should all be using assemblies for arbitrary widths, so there is some MATH table work to be done to make this work for some of the marks as already working for the arrow marks.

[tiroj]

Added horizontal assemblies for

U+034D | underleftarrow | COMBINING LEFT RIGHT ARROW BELOW
U+20D0 | overleftharpoon | COMBINING LEFT HARPOON ABOVE
U+20D1 | overrightharpoon | COMBINING RIGHT HARPOON ABOVE
U+20EC | underrightharpoondown | COMBINING RIGHTWARDS HARPOON WITH BARB DOWNWARDS
U+20ED | underleftharpoondown | COMBINING LEFTWARDS HARPOON WITH BARB DOWNWARDS

in MATH table dev for 2.20

[tiroj]

Updated sources and fonts delivered to STI Pub for testing. Issue reopened pending public release.