Technically "correct" you mean; if there was an unicode character that filled all the screen with the color black it would not matter if it were technically "correct", usability correct is more important.
If stacking diacritics are a legitimate part of a language and they change the meaning of words or characters, then it's more important that the content be correct than the "usability."
In fact, if a person can't read it or reads it incorrectly because parts of characters are hidden, then it's not very usable.
Hypothesizing about a Unicode character that fills the screen with black is a nonsensical straw man, because it makes no sense in "real world" written languages, so there would never be a Unicode character for it.
False; diacritics commonly used in the real-world such as "´¨`" fit inside the same space as the characters, this successive chain of diacritics is never used in real-world texts except for very few obscure cases. Plus the implementation of UTF8 should include the line-height required for the correct displaying of the character if they really believe the displaying of obscure characters is more important than usability.
