وَكَشْفُ رُبُعِ عُضْوٍ مِنْ أَعْضَاءِ الْعَوْرَةِ يَمْنَعُ صِحَّةَ الصَّلَاةِ
Exposure of one-quarter of an Awrah member reaches the invalidating threshold when it lasts for the legally relevant duration. The commentary explains accidental exposure in terms of the time needed to perform a pillar. Exposure below one-quarter does not reach this threshold.