Previous frame inner to this frame(corrupt stack?)

[wysota]

even hide() and show()?

Yes.

I can do it in MFC.

This is not MFC. GUI operations are not thread safe (especially in Qt3). Use custom events to post a proper event to the main thread instead.
In your case this is probably as simple as

QApplication::postEvent(mywindow, new QShowEvent());

QApplication::postEvent(mywindow, new QShowEvent());

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.

BTW. Does it make sense to issue "hide()" from within closeEvent? What do you need it for? close event hides the widget by itself... Just remember to call its base class implementation.