QStackedLayout/StackAll: Z-order changes when gaining focus from opened QComboBox by clicking through a mask of the widget in front

QStackedLayout Z-order changes when the backmost element is focused while QComboBox is open in a dialog.

Steps to reproduce:

• Create a stacked layout with StackAll, have a widget with a mask in front.
• Open a nonmodal dialog which contains a QComboBox
• Open the QComboBox
• Click (through mask) on the backmost element in main window's stacked layout

Expected result:

• Main window gets focus, stacked layout z-order does not change

Observed result:

• The clicked backmost element pops on top, hiding the other elements in stackedlayout

In the sample program clicking the black box opens a popup, after which you need to open the combobox. While combobox is opened, click the red background to observe the problem.