I don't have access to my test install of XF ATM due to a HDD issue on my pc, but could somebody check this out for me:
Revoke the delete threads by self permission and allow delete posts by self permission, then delete the first post in a thread - is the thread deleted or is a permission denied error thrown?
Revoke the delete threads by self permission and allow delete posts by self permission, then delete the first post in a thread - is the thread deleted or is a permission denied error thrown?