In Qt 4.8.7 (the version of Qt 4 we're transitioning from), QDir::cleanPath works as-expected when switching servers for UNC paths:

QDir::cleanPath("//server1/../server2/path/to/file.txt") == "//server2/path/to/file.txt"

Under Qt 5 (5.10.1 to be exact), this behaviour no longer works, QDir::cleanPath will no longer go below the server-name, resulting in the path being unchanged - unfortunately this unchanged path does not work with anything (e.g. QFileInfo(path).exists() returns false, where as under Qt4 it would return true).

This makes handling multiple files difficult when they're spread across different network locations (our typical use case is converting absolute <-> relative paths, where one file may be on a different network share to the reference file).