Leaving the Haftorah open until the concluding blessings are recited:
The Sefer Nevim [i.e. Haftorah; Chumash] is not to be removed from before the Maftir until after he completes the recital of the after blessings in order so he sees [the Haftorah] and says the blessing over the [words that he read for] Haftorah.
May one close the Haftorah book prior to reading the after blessings?
It is permitted to close the Sefer Haftorah prior to the blessings, so long as one does not remove the Sefer from the area. However, some Poskim rule it is initially best to leave the Sefer open to the Haftorah until after the blessings are recited.
 Admur 284/12; M”A 284/6; M”B 284/12; Shivlei Haleket 80 in name of Geonim; Tanya 16; Mateh Moshe 455; Hagahos Maimanis
 M”B ibid
 The reason: This is similar to the reading of the Torah in which case we rule the Sefer Torah may not be removed until the after blessing [Mateh Moshe ibid]
 Elya Raba 284/10 based on implication of M”A and Admur ibid; Kaf Hachaim 284/34The reason: As even by a Sefer Torah, one may close the Sefer prior to the after blessing, and the prohibition is only against removing it. [Elya Raba ibid]
 Shaareiy Efraim 9/34; See “The Laws of Purim” Chapter 7 Halacha 9 regarding a dispute in when the Megillah is to be rolled up, after or before the blessing of Harav Es Riveinu
 The reason: As when reading from a Chumash, it is not recognizable that one is saying the blessing over the Haftorah if he closes it. [Pischei Shearim 9/14]