When is an ASCII space (0x20) not a word separator?
When it's followed by a combining mark (e.g., COMBINING ACUTE ACCENT a.k.a. Unicode character 0x301).
According to ATSUI, anyway. Uniscribe disagrees and refuses to combine marks with space characters. It will allow combination if you stick a ZWJ (0x200D) in between. Gah!
We've also discovered that ATSUI's font fallback machinery often likes to choose different fonts for the mark and the character it combines with. Madness!
This is life working on Web browsers: the environment is so complex, any assumptions you make will be violated sooner or later.