Looking at the code of Filesystem::makePathRelative method I have found several issues:

• Stripping of drive letter for arguments is done regardless of host OS but directory separator normalization is done only for Windows hosts.
• There is no drive check for Windows paths, hence for the following case result will be invalid: (new Filesystem())->makePathRelative('C:/aa/bb/cc', 'D:/aa/dd/..') produces 'bb/cc/'
• (new Filesystem())->makePathRelative('/aa/bb/cc', '/aa') produces 'bb/cc/' which is wrong since '/aa/bb/cc' can be the file path, the presence of trailing slash in the result should be based on presence of trailing slash in the $endPath argument.