ComboBox popup cannot have focus

QQuickComboBox assumes that its popup window will never get the keyboard focus: all keys are handled by the QQuickComboBox itself, and if the QQuickComboBox item loses focus (for instance, because it's moved to the popup), then it hides the popup.

This makes it impossible to properly style the popup window in some styles (where the focus is visually indicated) and makes it harder for clients to use custom popups with different key handling.

I plan to submit a patch where the popup window is not closed, if the focus is being moved to it or one of its children. Feel free to assign this bug to myself.