There was a bytes / array elements confusion in the code that prints
out the input and output Unicode strings when a test fails. It was
using a loop with index variable 'pos', which was used as an array
index, but incremented by sizeof(a character) each time, leading to
only every fourth character actually being printed.
I assume this is leftover confusion from when I hadn't quite decided
whether to abuse the char-based strbuf for these Unicode character
buffers, or make a specialist type.
Discovered when I reached for this test program just now in order to
manually decompose a Unicode string. It doesn't have a convenient CLI
for that, but it was a thing I already knew where to find!
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