Qt takes wrong XDG_RUNTIME_DIR when running program with su -c

Following is a modified snippet from src/corelib/io/qstandardpaths_unix.cpp

const uint myUid = uint(geteuid());

qDebug() << "myUid" << myUid; 

QFileInfo fileInfo; QString xdgRuntimeDir = QFile::decodeName(qgetenv("XDG_RUNTIME_DIR"));

qDebug() << "xdgRuntimeDir" << xdgRuntimeDir;  fileInfo.setFile(xdgRuntimeDir); 

qDebug() << "fileowner:" << fileInfo.ownerId();

// "The directory MUST be owned by the user"

if (fileInfo.ownerId() != myUid) {

  qWarning("QStandardPaths: wrong ownership on runtime directory %s, %d instead of %d", qPrintable(xdgRuntimeDir), fileInfo.ownerId(), myUid);

}

If I build and run this as normal user on Linux, the output is:

$ ./test

myUid 1000
xdgRuntimeDir "/tmp/xdg-runtime-guest/"
fileowner: 1000

However, if I run it with su:

$ su -c ./test

myUid 0
xdgRuntimeDir "/tmp/xdg-runtime-guest/"
fileowner: 1000
QStandardPaths: wrong ownership on runtime directory /tmp/xdg-runtime-guest/, 1000 instead of 0