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.
If month change notification was posted async there was a possibility
to get a mousePressEvent() with already new Context field values, but
with old _selected value. Those two could be inconsistent leading to
an assert violation in (_selected + _context->daysShift() >= 0).
John Preston
23rd