C, not C++. Just want to say that upfront.
So, we maintain an old libre open source game called Project: Starfighter, originally developed by Parallel Realities in 2003. As you might imagine, it didn't originally support Unicode; we added unicode support (or more specifically UTF-8 support) in later.
Unfortunately it seems one aspect of the Unicode system we implemented is broken: line wrapping. We thought we had this solved with the Pango library (the only one we could find), which reports where line breaks can be made… but it reports on a grapheme cluster level, not at a code …