As far as i could tell this issue only exists on Windows and only with Qt 5.9.1 (Qt 5.9 seems fine).

A quick way to test it is to add a QComboBox to the Qt Window Flags example, for instance by adding the following to the PreviewWindow constructor:

comboBox = new QComboBox(this);

comboBox->setEditable(true);

layout->addWidget(comboBox);

Then, add a few items to the combobox manually and check the "stays on top" box.

Using the dropdown menu should result in some (or all, depending on its position) of it being hidden behind the window.