When user defines some class/struct that wants to be used as signal/slot parameter
lets say
class MyStruct
{
public:
QString mMember;
};

then if it is passed between slot and signals from two differen threads, then the slot function will not called

In attachment sample code that reproduces the issue

slot
void MyWidget::someStructSlot( const SomeStruct & )
{
cout << "received someStructSlot" << endl;
}

will never be called,

The issue does not occur when slot/signal are from the same thread