There is a potential crash problem when hiding/closing widgets in a certain order on Mac OSX.

Calling QWidget::hide crashes because of an invalid "qt_mouseover" variable defined as
extern QPointer<QWidget> qt_mouseover; //qapplication_mac.mm:line 190

qt_mouseover is set in the QWidget::hide_sys function as a result of QApplication::widgetAt(QCursor::pos());

The problem occurs when mousing over a widget as it is hiding. A previous call to hide_sys executes the QWidget::destroy code (the mouse is over a widget which is half dead) twice.

That means that QObjectPrivate::clearGuards has been already called for that particular widget and qt_mouseover holds an invalid pointer.

The next call to hide_sys tries to do something with qt_mouseover in dispatchEnterLeave and crashes.

Now qt_mouseover still holds a reference to the already deleted DockWindow widget. The next call to hide_sys (doesn't matter for which widget) crashes

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_PROTECTION_FAILURE at address: 0x00000001
0x050aad7d in QWidgetPrivate::hide_sys (this=0x1f0c6d80) at kernel/qwidget_mac.mm:3398

(gdb) bt
#0 0x050aad7d in QWidgetPrivate::hide_sys (this=0x1f0c6d80) at kernel/qwidget_mac.mm:3398
#1 0x051590f5 in QWidgetPrivate::hide_helper (this=0x1f0c6d80) at kernel/qwidget.cpp:6804
#2 0x05162bd7 in QWidget::setVisible (this=0x1f0c6d30, visible=false) at kernel/qwidget.cpp:6984
#3 0x003c66d4 in QWidget::hide (this=0x1f0c6d30) at qwidget.h:477