fix: only apply Windows path-equivalence rules on Windows

claim_key folded backslashes into forward slashes on every platform. On POSIX a
backslash is an ordinary filename character, so "a\b.mp4" and "a/b.mp4" are
different files that would have compared equal -- a false collision, and a
rename the caller never asked for.

Case folding was already Windows-only; make the separator handling match and
compare byte-exact elsewhere.

The unit test asserted the Windows rules unconditionally and so failed on the
macOS runners. Split it: Windows keeps the equivalence assertion, POSIX asserts
the two paths stay distinct.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: summeroff

obs-studio-server/source/osn-file-output.cpp

@@ -29,16 +29,20 @@
 // as a unit -- the manager's own lock is dropped when for_each returns, so it is not enough.
 static std::mutex s_filenameClaimMutex;
 
-// Windows paths are case-insensitive and accept either separator, so compare on a folded key
-// while the claim itself keeps the exact string the muxer was given.
+// Windows paths are case-insensitive and accept either separator, so compare on a folded key while
+// the claim itself keeps the exact string the muxer was given. Elsewhere the comparison is exact:
+// on POSIX a backslash is an ordinary filename character, so folding it into a separator would make
+// two different files look like one.
 static std::string claim_key(const std::string &path)
 {
+#ifdef WIN32
 	std::string key = path;
 	std::replace(key.begin(), key.end(), '\\', '/');
-#ifdef WIN32
 	std::transform(key.begin(), key.end(), key.begin(), [](unsigned char c) { return (char)tolower(c); });
-#endif
 	return key;
+#else
+	return path;
+#endif
 }
 
 // Caller must hold s_filenameClaimMutex.

obs-studio-server/tests/test-osn-file-output.cpp

@@ -58,11 +58,22 @@ TEST_CASE("FindBestFilename avoids a path claimed by another live output", "[fil
 		REQUIRE(resolve(wanted, second.get(), false, true) == "C:/osn-test/2026-08-08 14-30-12 (2).mp4");
 	}
 
+#ifdef WIN32
+	// Windows only: separator and case are not part of path identity there. On POSIX a backslash
+	// is an ordinary filename character, so these really are different paths.
 	SECTION("separator and case differences do not evade the check")
 	{
 		REQUIRE(resolve(wanted, first.get()) == wanted);
 		REQUIRE(resolve("C:\\OSN-Test\\2026-08-08 14-30-12.mp4", second.get()) == "C:\\OSN-Test\\2026-08-08 14-30-12 (2).mp4");
 	}
+#else
+	SECTION("paths differing only by separator are distinct files")
+	{
+		REQUIRE(resolve(wanted, first.get()) == wanted);
+		const std::string backslashed = "C:\\osn-test\\2026-08-08 14-30-12.mp4";
+		REQUIRE(resolve(backslashed, second.get()) == backslashed);
+	}
+#endif
 
 	SECTION("the name is reusable once the first output stops")
 	{