On OS X, when you place a QComboBox (and presumably some other controls too) on a widget with the Qt::Popup window flag set, the drop-down menu of the combo box appears behind the widget. This is fundamentally wrong, as the parent of the combo box is the widget below which the menu is being drawn. This appears to be related to mouse events, as invoking the ComboBox's menu by hitting Space when it has the focus behaves correctly; it only goes wrong when you click on the ComboBox to show its menu.

Change 534ba3c7314820604ba5aeeffa6051c91e7c1d09 is the change that introduced this problem.