I disagree.

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.