Notes & justification
Abū Jaʿfar makes a light saktah on the nūn of نٓ, without taking breath, and the pause entails pronouncing the nūn clearly (iẓhār). The nūn of نٓ is assimilated, with ghunnah, into the wāw of وَٱلْقَلَمِ by Ibn ʿĀmir, Shuʿbah, al-Kisāʾī, Yaʿqūb and Khalaf in his own choice, and by Warsh with khulf from him; everyone else sounds it clearly, and that is Warsh's second wajh. The Shāṭibiyyah line gives the iẓhār of Yā-Sīn and Nūn to its code-group and names the khulf from Warsh in Nūn; the first Durrah line prescribes Abū Jaʿfar's saktah on the separate opening letters; the second gives the idghām of Yā-Sīn and Nūn to Khalaf and Yaʿqūb.
Yā-Sīn: pronounce it clearly for ʿan (Ḥafṣ), fatan (Ḥamzah), ḥaqquhu (Ibn Kathīr and Abū ʿAmr), badā (Qālūn); and Nūn likewise — and in it the khulf is from their Warsh.
Separate the letters of the opening sigla with a saktah — as in alif (the alif is Abū Jaʿfar's code); the second half-line turns to yakhdaʿūna and to the ishmām of al-Baqarah.
Assimilate the nūn of Yā-Sīn and of Nūn — fidan (Khalaf), ḥuṭ (Yaʿqūb); and the sīn before the mīm (of Ṭā-Sīn-Mīm) — fuz (Khalaf); in yalhath pronounce clearly — ud (Abū Jaʿfar); and in irkab it is widespread — fashā (Khalaf).
It is read with the nūn concealed. The justification: it is a sākin nūn, since the letters of the alphabet are built on sukūn, and a wāw follows it; the nūn is concealed before the letters of the mouth, and although this nūn is separate from the wāw it is treated as joined to what follows — so the nūn was concealed, for a nūn is concealed before the mouth-letters only when it is joined to them, and this behaves as joined. It is also read with iẓhār: iẓhār is the original and the analogical form, because the letters of the alphabet are taken as separate from what follows, for a reason given more than once; hence the nūn must be sounded plainly. (al-Mūḍiḥ 3:1287)