Agreed. The only thing that VS-15 is supposed to do is give the emoji a "text presentation", which the spec suggests as "black & white". And every example they show of a text presentation is exactly the same width as the emoji presentation.
Changing a character's width from wide to narrow after it has already been output is fraught with problems for a terminal. Imagine trying to write a "narrow" text presentation emoji in the bottom right corner of the screen. You'd think it should fit, but the emoji is received before the VS-15 selector, and that doesn't fit, so the terminal is forced to wrap the text, triggering a scroll of the entire page. By the time the VS-15 arrives, there's no way to undo all of that.
And for another example, try using IRM (insert replace mode) to insert a text presentation emoji in the middle of some existing text. If it was really narrow, you'd expect it to insert enough space for just one character, but it actually inserts two spaces, only occupying one of them, and then leaving an unexpected gap. And the more of these "narrow" characters you insert, the bigger the gap becomes.
VS-16 changing a text presentation character from narrow to wide doesn't share these problems. And that behaviour is supported by the spec text, which says that emoji should generally have a square aspect ratio. And at one time the East Asian Width spec specifically mentioned VS-16 making narrow characters wide (but said nothing about VS-15 making wide characters narrow).
This was also an option on many "Glass TTY" terminals - it was called the margin bell - and even some modern terminal emulators still have that option. The exact semantics vary, but it's usually triggered when entering content around 8 characters from the right margin.
> "ANSI" is what people call it when they are working from paltry and incomplete samizdat doco of how this stuff works
People just use "ANSI" as a shorthand for ANSI X3.64-1979. And that was the standard that DEC used for their VT100+ range of terminals, which in turn became the de facto standard from which most modern terminal emulators are derived. If you read the DEC documentation, you'll find many references to "ANSI standard", "ANSI controls", "ANSI colors", etc. I don't think this is because they were ignorant of the subject matter, considering that they were members of the committee that produced that standard.
And ECMA-48 is essentially just the European equivalent of ANSI X3.64, and was developed in parallel. But obviously an American company like DEC or Microsoft would more likely be working from the American version of the standard rather than the European one.
Changing a character's width from wide to narrow after it has already been output is fraught with problems for a terminal. Imagine trying to write a "narrow" text presentation emoji in the bottom right corner of the screen. You'd think it should fit, but the emoji is received before the VS-15 selector, and that doesn't fit, so the terminal is forced to wrap the text, triggering a scroll of the entire page. By the time the VS-15 arrives, there's no way to undo all of that.
And for another example, try using IRM (insert replace mode) to insert a text presentation emoji in the middle of some existing text. If it was really narrow, you'd expect it to insert enough space for just one character, but it actually inserts two spaces, only occupying one of them, and then leaving an unexpected gap. And the more of these "narrow" characters you insert, the bigger the gap becomes.
VS-16 changing a text presentation character from narrow to wide doesn't share these problems. And that behaviour is supported by the spec text, which says that emoji should generally have a square aspect ratio. And at one time the East Asian Width spec specifically mentioned VS-16 making narrow characters wide (but said nothing about VS-15 making wide characters narrow).