Would you be open to using a tracking issue or project board for the font work (sample categories: draft, waiting on other PRs, ready for review, please review first)?

I think breaking everything up into small PRs is fantastic but I'm finding it a bit overwhelming to keep track of the order for reviewing things (and figure I'm not alone here) and how these PRs relate to each other.

Sure, I'll do that. I'm sorry for the mess, but this is basically me trying to remember all the patches or even just ideas I have accumulated over the years that went nowhere because they would thrash all the baseline images, and that I now try to present given that I see an opportunity with the FreeType upgrade. Also, some of them are notfully complete patches but things that I believe can be made complete with relatively little effort (for some value of "little effort"...) simply because I do not have the time now to finalize them but want to put them up for discussion as the underlying idea should be clear.

Edit: seethe wiki; attn@QuLogic as well too.

I'm sorry for the mess, but

No apologies needed, I commend you for taking advantage of the freetype upgrade. And thanks for the wiki, though some reason can't link cleanly☹️

link fixed

Edit: fraction bar rendering has been added too.

A few warnings from compiling locally:

[2/4] Compiling C++ object src/ft2font.cpython-313-x86_64-linux-gnu.so.p/ft2font_wrapper.cpp.o../../src/ft2font_wrapper.cpp: In lambda function:../../src/ft2font_wrapper.cpp:1785:22: warning: unused variable ‘error’ [-Wunused-variable] 1785 |             if (auto error = FT_Load_Glyph(face, idx, static_cast<FT_Int32>(flags))) {      |                      ^~~~~../../src/ft2font_wrapper.cpp:1788:22: warning: unused variable ‘error’ [-Wunused-variable] 1788 |             if (auto error = FT_Render_Glyph(face->glyph, FT_RENDER_MODE_NORMAL)) {      |                      ^~~~~../../src/ft2font_wrapper.cpp: In lambda function:../../src/ft2font_wrapper.cpp:1804:26: warning: unused variable ‘error’ [-Wunused-variable] 1804 |                 if (auto error = FT_Glyph_To_Bitmap(&g, FT_RENDER_MODE_NORMAL, &origin, 1)) {      |                          ^~~~~

This is semi-intentional: if these checks fail (error > 0) then the error should really be raised using throw_ft_error, not with a plain raise as I currently do, but throw_ft_error is not visible from ft2font_wrapper.cpp so some code needs to be moved around, e.g. make throw_ft_error an inline function defined in ft2font.h, or move the new implementations into real C++ FT2Font methods in ft2font.cpp and add wrapper lambdas in ft2font_wrapper.cpp, or (would be my preferred choice) switch error checking to the FT_CHECK macro used in mplcairo (see mplcairo's macros.h).
I left the warning as is just to keep this as a side discussion point, but it can trivially be suppressed by not capturing the error value too.

On a side note, I'm in favour of this in general as removing the intermediate buffer will make emoji/colour glyphs easier since they won't need to go through it.