Is it considered makruh (disliked) or bad manners to extend one's legs in the mosque, whether or not there is a Quranic cabinet present?

There is no dislike (makrooh) in stretching one's legs inside the mosque, and this is permissible for both those in i'tikaf (spiritual retreat) and others. As for stretching one's legs towards the Qibla, its ruling has been detailed in a previous fatwa. However, regarding stretching one's legs towards the direction of the Mushaf (copy of the Quran), scholars have differed: some have prohibited it, while others have deemed it disliked. Therefore, one should avoid stretching their legs towards the Mushaf's direction as much as possible.