Regression was introduced in d0e854e9d8.
Each time you hide() a widget its mousePress state is reset and
mouseMoveEvent() / mouseReleaseEvent() handlers are never called.
So you can't toggle (hide + show) widgets that are mouse-pressed.
Fixes#4802.
base::lambda -> Fn (type alias for std::function).
base::lambda_once -> FnMut (type alias for base::unique_function).
base::lambda_guarded -> crl::guard.
base::lambda_call_type_t -> crl::deduced_call_type.