What is the ruling of the Sharia on some people talking about worldly matters in the mosque, some of them shaking hands during a religious lesson, and some of them leaving the mosque?

Whoever enters the mosque must occupy himself with that for which mosques were built: the remembrance of Allah, prayer, reading the Quran, and disseminating knowledge. It is not permissible to disturb those praying or studying knowledge by talking or greeting loudly. The Prophet, peace and blessings be upon him, warned against clamor and raising voices in mosques, because that is a disrespect to mosques. As for what is outside the mosque, there is no harm in it.