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.
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.