You don't know whether it's an unterminated raw string (the usual case)
until you've scanned for a potential closing delimiter.
Is C++ syntax so ambiguous? Can R"( mean something else?
No it has nothing to do with the language's syntax.
It's just a UI choice: Alan likes to highlight the opening string
delimiter of unterminated strings [ Not sure why CC-mode doesn't do the
same for unterminated comments, tho maybe it's just an oversight. ]
I think this should not vary between major modes, IOW it should be
a separate feature that works on all major modes.