The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in discussing the laws of Purim (Tesaveh, 4), writes that it is customary for the one who reads the Megilla in the synagogue to unravel the Megilla completely, from the beginning to end, before he starts reading. He then rolls the Megilla closed upon completing the reading. Those who are listening to the reading, by contrast, should roll the Megilla open as needed over the course of the reading, rather than unravel the scroll entirely at the beginning. This custom is based upon the comment of the Tur (Rabbenu Yaakob Ben Asher, 1269-1343), who writes "Poshet Ve’koreh" – that the reader should unravel the scroll and then read.
However, several sources indicate that the custom in Halab (Aleppo, Syria) was for the reader, too, to unravel the scroll as needed over the course of the reading, rather than unravel it completely before starting to read. This custom is recorded in the work Hochma U’musar (244), citing from a work called Bet David (Siman 481). It is also mentioned in the book Derech Eretz, which documents the customs practiced by the Jews of Halab (listen to audio recording for precise citation). This custom follows the implication of the Shulhan Aruch (Orah Haim 680:17), who writes (based on the Rambam), "Koreh U’poshet" – that the reader reads and unravels the scroll over the course of his reading.
The only difference between the reader and the congregation according to this custom is that the reader leaves the scroll open as he reads. Whereas the congregants who are following from a scroll roll their scrolls closed as the reading progresses, leaving open only the part they need to follow along, the reader leaves the scroll open as he reads. He does not roll it closed until the completion of the Megilla, before reciting the concluding Beracha.
Summary: According to the tradition of the Jewish community of Halab (Aleppo), the one who reads the Megilla in the synagogue does not open the entire Megilla before he begins reading. Rather, he opens the scroll as he reads, and leaves it open until after the completion of the reading. The congregants who follow along from a Megilla should open the scroll over the course of the reading but close it as the reading progresses, leaving open only what they need to follow along.
