> By default, Emacs already treats the ellipsis as a Narrow character, and our current idea of "context" is the value of language-environment, when the font information is not available.
I'll try to clarify my opinion a bit more.
What I meant by default was default in the CJK language environment and the default width of the ambiguous characters in CJK environment should be narrow. Current emacs changes the width of ambiguous characters to wide if the user activates the CJK environment. The unicode standard recommendation is set the width narrow at unclear circumstances but emacs changes the width to wide even if it can't know what font is currently used. For that reason, I don't think such behavior is aligned well with the unicode standard. Furthermore, The majority of the default width of those characters in the CJK environment is narrow on contemporary implementation of the terminals from my limited experience. However, Considering the emacs package ecosystem, current emacs behavior is ok as long as there's an easy option for changing such values.
I hope this makes sense.