Is it permissible to stretch (extend) one's legs inside the mosque, and is it obligatory to forbid someone from doing so?

Extending the legs in the mosque or towards the Qibla is considered disliked by some scholars, as there is no explicit legal proof for its dislike. However, it is preferable to avoid it. There is no need to forbid the doer of such an act, as the action is not unlawful, but there is no harm in advising them.