See attached code,
If you:
1 add a widget to a visible layout
2 remove that widget
3 set that widget's parent to NULL

the widget will appear as a top level window, because in step 1, QLayout::addChildWidget() queues a call to QWidgetPrivate::_q_showIfNotHidden(), which calls setVisible(true) while the parent is NULL, hence the widget becomes a top level widget.

The code explains the motivation for wanting to do it in the first place, the comments copy-pasted are:
// The purpose of this code is to try and identify the correct way to:
//
// * Add a widget (A) to a visible layout (L), and then immediately remove it
// * Delete the layout's widget-parent (P), and ensure widget-A is NOT deleted as if it were still a child of P.
//

// Approach 1 - do nothing special,
// but this will result in a being destroyed

// Approach 2 - set parent to NULL,
// but this will result in a becoming a window of its own

// Approach 3 - set parent to a QWidget that is never shown
// this works but this solution was only discovered after diving into QT code, it is not obvious from reading the documentation