Scheduler delays runnable when added outside component lifecycle

[intendednull]

Problem

#1903 introduced a bug that caused messages sent to an agent outside a component's lifecycle to be delayed until a component lifecycle event is triggered.

Yewdux uses an agent bridge for generating flexible callbacks. If a message is sent immediately inside a component's lifecycle, it is executed as expected:

        self.bridge()
            .borrow_mut()
            .send_service(ServiceRequest::ApplyOnce(Box::new(f)))

However when a message is sent from a inside a callback it will not be executed.

        let bridge = Rc::clone(self.bridge());
        let f = Rc::new(f);
        Callback::from(move |_| {
            bridge
                .borrow_mut()
                .send_service(ServiceRequest::Apply(f.clone()))
        })

Only when a component lifecycle event is triggered will the bridge message then be executed.

Expected behavior
Bridge messages should be executed normally when sent from a callback.

Environment:

• Yew version: master
• Rust version: nightly

Questionnaire

• I'm interested in fixing this myself but don't know where to start
• I would like to fix and I have a solution
• I don't have time to fix this right now, but maybe later