forgot that:

Le jeudi 28 octobre 2021, 08:53:21 CEST Eli Zaretskii a écrit :
> IOW, find a real-life case with several diacritics on the same
> character, and then let's talk.  The example you provided makes no
> sense in real life, and the correct reaction of the Emacs display
> engine to that nonsensical example is to show just one diacritic.

“Vietnamese (Vietnamese: tiếng Việt) is an Austroasiatic language” from 
https://en.wikipedia.org/wiki/Vietnamese_language (which firefox displays 
just fine, screenshot attached).

For some reason, for me, within emacs, this displays bad: the e is 
oversmall, while using combining diacritics displays right: “tiếng Việt”.

Whatever the way, Qt displays that incorrectly just the same.

Screenshots attached (but do you look at them?)