#182 Improve support for stacked diacritics

[Anonymous]

I'm in contact with the developer of the New Computer Modern font
because I asked him to add support for stacked diacritics in his font.
After various tests and fixes he was able to make diacritics that stack properly when the font is viewed in FontForge, but the LaTeX output still has some rendering issues, so we believe that they are caused by bugs in the compiler. I attach two pdfs, one containing the XeLaTeX's output, one containing LuaLaTeX's output for comparison, and I attach the source file as well.
I would like to emphasize some test result:
- test 5: XeLaTeX does not show the diacritic above the a, while LuaLaTeX does so correctly;
- test 6: XeLaTeX mashes the upper diacritics together and prints the arrow below a too much to the left, while LuaLaTeX produces the correct output;
- test 9: XeLaTeX mashes together the first two lower diacritics, while LuaLaTeX shows the correct output;
- test 10: in both occasions the diacritics are stacked in the same order, even though they are in different order in the source;
- test 11: the last diacritic should stack on top of the second-to-last, not on the side of the first one;
- test 12: XeLaTeX mashes together the two diacritics below the letter, but one should go at the top and one at the bottom (kind of like Times does in the same test).

These tests were made on TeXLive 2020 for Windows, updated on 2021-03-19.
Thank you for your attention.

[Anonymous]

Update: there was a bit of confusion about character U+0315 and its behavior, so ignore the results of test 10, 11 and 12 for New Computer Modern; XeLaTeX is working properly because Times shows the proper behavior of the diacritic in these tests.

[Anonymous]

Update 2: Antonis Tsolomitis recently uploaded a new version of New Computer Modern, which contains the stacking diacritics, to the CTAN. If you want to reproduce the bug, use those fonts instead of the one contained in mwe.zip.