Qt 不支持在Mac中的鼠标中键(滚轮)拖动

有人问起这个问题，虽然我不用Mac但是代码中有证据表明，所言属实。

503 Qt::DropAction QDragManager::drag(QDrag *o)
504 {
505 if(qt_mac_in_drag) { //just make sure..
506 qWarning("Qt: Internal error: WH0A, unexpected condition reached");
507 return Qt::IgnoreAction;508 }
509 if(object == o)
510 return Qt::IgnoreAction;
511 /* At the moment it seems clear that Mac OS X does not want to drag with a non-left button
512 so we just bail early to prevent it */
513 if(!(GetCurrentEventButtonState() & kEventMouseButtonPrimary))
514 return Qt::IgnoreAction;

除非重新设置你的kEventMouseButtonPrimary为中键，否则被忽略。