On a point of standards interpretation:

Le sam. 6 janv. 2024 à 11:56, Corentin <corentin.jabot_at_[hidden]> a écrit :

> It is very surprising to me that ZWJ is opt-out rather than opt-in given
> […] the fact supporting them requires implementation of TR39 3.1.1
> <https://www.unicode.org/reports/tr39/#Joining_Controls>.
>
Supporting these characters requires no such thing.

It is indeed the case that most of this section used to be in UAX #31 with
requirement UAX31-R1a.
That requirement was removed to UTS #39 for the reasons mentioned above (it
falsely suggested that default identifiers do not otherwise contain
invisible characters, and its complexity incentivized implementers to not
allow these linguistically important characters in identifiers, even though
they allow the other less useful default ignorables).
Note that the application of the contextual checks in Section 3.1.1 of UTS
#39 is described in the example in Section 5.1.3 of UTS #55
<https://www.unicode.org/reports/tr55/#General-Security-Profile> as a
mitigation to a *usability* issue; the security issue is dealt with as part
of the more general confusable detection, which ignores default ignorables
<https://www.unicode.org/reports/tr39/#Confusable_Detection>.