Is a woman considered pure upon the cessation of bleeding before Fajr, or upon seeing dryness after waking up? And in this case, is she required to fast or may she break her fast?

If a woman sees her menses an hour before Fajr, then doubts the time of purity upon waking, the default is that purity occurred after Fajr. This is because the principle is "attributing a new occurrence to its nearest possible time." Since the condition for the validity of fasting is the occurrence of purity before Fajr, her fast is not valid, and she must make up that day. However, if a menstruating woman becomes pure during the day, the view of the Malikis and Shafi'is, which is the stronger opinion, is that she is not obligated to refrain from eating and drinking for the rest of the day, unlike the Hanafis and Hanbalis who require her to do so.