KTreeWidgetSearchLine docs say that event() is reimplemented "for internal reaons" and that "API is not affected". However, this re-implementation steals the handling of the Return key from KLineEdit, meaning that the returnPressed() signal will not be emitted when using a KTreeWidgetSearchLine. I think this should be mentioned somewhere, if not a switch provided to disable.
(removing myself from the cc list, as i've nothing to do with that class, i'm author of the proxy search line class, but not this one).