The Poskim discuss the proper custom regarding unraveling the Megillah on Purim. Maran (680:17) and the Rambam state that "One reads and unravels," clearly implying that the Megillah is unrolled as it is read, and not before. On the other hand, the Tur (Rabbenu Yaakob Ben Asher, 1269-1343) records that one should first unravel the entire scroll and then begin reading. The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) rules in accordance with the Tur regarding the Ba'al Koreh-the reader.
Nevertheless, the custom of Halab, as recorded in Derech Eres, is in accordance with the Shulhan Aruch and the Rambam, even regarding the Ba'al Koreh. He unravels the scroll as he reads, leaving it completely unrolled until he completes the reading. Then he rolls it back up and makes the final Beracha. The rest of the congregation also unravel their Megillahs as the reading progresses, but they do not leave it unfolded. Rather, they roll it back up on the other side as the reading progresses.
SUMMARY
The custom of Halab is for the reader to unravel his Megillah as he reads, leaving it completely unrolled until he completes the reading, at which points he rolls it back up, and then makes the final Beracha. The rest of the congregation also unravel their Megillahs as the reading progresses, but they do not leave it unfolded.
