Arabic text is broken in different ways.

// With this font the letters are overlapping each other in a very bad way
QApplication::setFont(QFont("KFGQPC Uthman Taha Naskh", 72));
QTextEdit *text = new QTextEdit(QObject::trUtf8("الرَّحْمَنِ"));

// With this font the letter Ha and Meem should be connected
// Attached is a file with a red circle where the sapce is wrong
QApplication::setFont(QFont("Amiri", 72));
QTextEdit *text = new QTextEdit(QObject::trUtf8("رحْمن"));

Those fonts are free to use. I created a package for them at
https://aur.archlinux.org/packages/ttf-amiri/
https://aur.archlinux.org/packages/ttf-qurancomplex-fonts/