Unicode defines the width of characters. This is what the POSIX functions like wcwidth refer to. eg https://www.unicode.org/reports/tr11/
So you shouldn't need knowledge of the type-face to know the size of the printed character. If other libraries are printing the characters and then extracting the width from that, then they're doing it wrong and you should look to use a different library instead.
The bigger problem I've run into is not different libraries making up their own width assumptions. It's been libraries built against outdated versions of the Unicode standard.
> The terminal emulator must handle a lot of RtL stuff to get it acceptable. In raw mode (where dimensions are knowable) you could almost leave it to the app, but outside raw mode it’s pretty obvious that the terminal has to be BiDi-aware. But even in raw mode, you really can’t treat it as you describe, because Arabic text rendering is rather complex, full of necessary ligatures.
I think RTL is a distraction from the actual point that we're discussing. But it's worth noting that there are already CSI sequences for enabling RTL behaviour in terminal emulators. It's not something I've personally implemented in my own projects because I have no personal need for it and have had no feature requests from people asking for it who could test the implementation's soundness (unlike Chinese characters, which have seen a reasonable degree of popularity in the projects I've built). But I'm sure some other terminals support it just fine since it was added decades ago as part of ECMA 48.
No, UAX #11 does not define the width of characters. It’s called “East Asian Width” for a reason. As the note in the Scope section begins:
> The East_Asian_Width property is not intended for use by modern terminal emulators without appropriate tailoring on a case-by-case basis. […]
It’s about a very specific typographical custom in a specific set of languages. Terminal emulators have piggy-backed onto it because it’s fairly close to what they want and definitely better than nothing, but if they limit themselves to this definition, they do a bad job.
> you shouldn't need knowledge of the type-face to know the size of the printed character
To see the most extreme example, look through Google Fonts for U+FDFD <https://fonts.google.com/?preview.text=%EF%B7%BD> and see the radical differences in aspect ratio (and size!). In some other scripts there are multiple ways of writing things which have quite different width characteristics. In English we have different accepted and understood letterforms for a, g, v, z and probably others, but they’re all roughly the same width. But this isn’t always true in other scripts. My name is written in Telugu as క్రిస్ (kri s), but that r curve can be drawn below or to the left (both seen among <https://fonts.google.com/?preview.text=%E0%B0%95%E0%B1%8D%E0...>), and even sometimes a little to the right; and you don’t actually have to write it with a conjunct, క్ రిస్ (k ri s) means the same thing and there used to be some fonts or apps that wouldn’t render conjuncts. (Encodingwise, I inserted a ZERO WIDTH NON-JOINER after the {k, suppress vowel} and before the {r, -i} to simulate this effect. And then HN turned the ZWNJ into a space. Sigh. I should ask them once again to stop doing that.) Of these three different legitimate renderings of the same text, one works in two cells, one wants two and a half, one needs three.
> No, UAX #11 does not define the width of characters.
It's literally there in the doc
> It’s called “East Asian Width” for a reason. As the note in the Scope section begins:
Yes. I didn't claim it defines terminal cell widths. There's a reason you need a library to translate that into cell widths. What I said was you don't need to be pratt about with printing because Unicode does define widths and properly libraries pin against that (like POSIX does).
> Terminal emulators have piggy-backed onto it because it’s fairly close to what they want and definitely better than nothing, but if they limit themselves to this definition, they do a bad job.
That's a very convoluted way of agreeing with me :)
> To see the most extreme example, look through Google Fonts for U+FDFD <https://fonts.google.com/?preview.text=%EF%B7%BD> and see the radical differences in aspect ratio (and size!).
You're shifting goal posts again because you're no longer talking about fixed width typefaces. Terminals have to make some concessions for turning scripts (of any language) into fixed width. If that's an issue then you shouldn't be using the terminal to begin with. There are better standards for that, such as the web.
In short, nobody is going to disagree with you that Unicode is hard and full of edge cases. But the vast majority of the stuff you describe are either already solved, already have standards, or are already out of scope for terminals.