I'm not sure it is so common in UIs. Truncation is done by a single library function, so that's one case where it's used. Another case is for character wrapping, but that's fairly uncommon. I'm having trouble coming up with another case where it's used. Font shaping is done by a font shaping engine, which applies an enormous number of rules specific to the script in use. Text in a text editor isn't deleted according to grapheme cluster boundaries, and the text cursor doesn't fall on grapheme cluster boundaries either. These are all rules that change according to the script in use.
