The result should be "..". In fact, the current return value is quite strange that its getName(0) is also ..\. Seems like \ is not treated as a path separator.

The problem is inside relativize() of WindowsPath.java:

 416 // append remaining names in child
 417 for (int j=i; j<cn; j++) {
 418 result.append(other.getName(j).toString());
 419 result.append("\\");
 420 }

Here, when other.getName(j).toString() is empty, a backslash should not be added.

Another similar issue. Paths.get("D:").toAbsolutePath() becomes D:\xyz\ instead of D:\xyz. The WindowsPath::getAbsolutePath() method leaves a trailing slash when remaining is empty below.

 252 result = defaultDirectory + "\\" + remaining;