I don't think I agree with this. Always returning a promise from simulate seems to suggest the event system is somehow asynchronous (which it itsn't) and users of the library should always wait for the promise to resolve before proceeding with their tests.

For the use case you exemplify in the test you added, why not do this?

    it('should simulate promise events', (done) => {
      class Foo extends React.Component {
        ...
      }

      const wrapper = shallow(<Foo />);

      expect(wrapper.find('.clicks-0').length).to.equal(1);
      wrapper.prop('onClick')().then(() => {
        expect(wrapper.find('.clicks-1').length).to.equal(1);
        done();
      });
    });

ie. select the value of the onClick prop and call it directly

Events are not appropriate to model as Promises. Events happen multiple times; a promise resolves once.

Also, yes, events are synchronous in the browser.

You can wrap yourself with new Promise to avoid the done pattern.