Solution

//Current
this->dropdownMenu_.move(this->dropdownButton_->mapToGlobal(QPoint(0, this->dropdownButton_->height())));
this->dropdownMenu_.show();

//Potential solution #1 Synchronous
//no need for .move() anymore
this->dropdownMenu_.exec(QCursor::pos());

//Potential solution #2 Asynchronous
//no need for .move() anymore
this->dropdownMenu_.popup(QCursor::pos());