Run the named fixture in the local counter, then compare each result with the exact rule rather than an assumed universal character definition.
CRLF counts as one break
For one CRLF pair, WordCount replaces the pair with one LF before splitting. Standalone CR and standalone LF also become one break. The fixture one, two, three, four with mixed endings therefore has four word-bearing lines before its final break.
A final break creates a trailing empty line
Any non-empty input has at least one line. When it ends with a line break, splitting produces one additional trailing empty line, so the mixed fixture reports five total lines and four non-empty lines. This rule is explicit because editors differ in how they present a final newline.
Blank lines separate paragraphs
One ordinary line break stays inside a paragraph. One or more blank lines, optionally containing horizontal Unicode whitespace, start another paragraph. Whitespace-only input is rejected, so an old successful result cannot remain visible beside an empty form.