macOS: QComboBox Dropdown is instantly closed when displayed in a QToolBar

If a combo box is part of QToolBar that is shrinked to its lowest possible and then when combobox is clicked dropdown closes immediately.

Attached is the example to reproduce:

Steps:

• minimize the width of the MainWindow to the minimum
  => the toolbar should be shrinked, the 2 comboboxes should not longer be visible and the QToolbar Extension button should be visible.
• click on the QToolbar Extension button
  => the 2 comboboxes should becomes visible below the QLabel
• click on the leftmost ComboBox DropDown Button 
  => BUG: The DropDown list is not displayed but is instantly closed which does not let the user to select a value.
  In some cases the use cannot click anywhere in the UI after that and he as to press the ESC key to be able to interact with the UI again.