Adapt the test.

src/host/ft_host/API_FillOutputTests.cpp

@@ -10,6 +10,24 @@ class FillOutputTests
     BEGIN_TEST_CLASS(FillOutputTests)
     END_TEST_CLASS()
 
+    // Adapted from repro in GH#4258
+    TEST_METHOD(FillWithInvalidCharacterA)
+    {
+        VERIFY_WIN32_BOOL_SUCCEEDED(SetConsoleOutputCP(50220));
+        auto handle = GetStdOutputHandle();
+        const COORD pos{ 0, 0 };
+        DWORD written = 0;
+        const char originalCh = 14;
+        VERIFY_WIN32_BOOL_SUCCEEDED(FillConsoleOutputCharacterA(handle, originalCh, 1, pos, &written));
+        VERIFY_ARE_EQUAL(1u, written);
+
+        char readCh = 42; // don't use null (the expected) or 14 (the actual) to ensure that it is read out.
+        DWORD read = 0;
+        VERIFY_WIN32_BOOL_SUCCEEDED(ReadConsoleOutputCharacterA(handle, &readCh, 1, pos, &read));
+        VERIFY_ARE_EQUAL(1u, read);
+        VERIFY_ARE_EQUAL(0, readCh, L"Null should be read back as the conversion from the invalid original character.");
+    }
+
     TEST_METHOD(WriteNarrowGlyphAscii)
     {
         HANDLE hConsole = GetStdOutputHandle();