This view attaches a message filter to its parent window. That filter forwards mouse messages to the MDI view the mouse is over. That is necessary, because BeOS doesn't know anything about the z-order of the views and directs the messages to the first view it finds, that contains the mouse.

But during drag operations normally some view has used SetMouseEventMask to receive all mouse messages, regardless if the mouse is over that view. In that case you must disable the mouse redirect filter, because the filter would change the target again to that view the mouse is over.