.once modifier did'nt perform as expected if my event handler return null

[LKL2017]

Version

2.6.11

Reproduction link

https://jsfiddle.net/q0v6m8dr/1/

Steps to reproduce

1. bind a handler to an event which with a '.once' modifier
2. set null for the hanlder's return value
3. trigger the event, and trigger again

What is expected?

only the first trigger will invoke handler, under '.once' influence

What is actually happening?

subsequent trigger will also invoke handler beside the first one

[posva]

I'm unsure about this one since the condition is explicitly stated at https://github.com/vuejs/vue/blob/dev/src/core/instance/events.js#L36 but I don't know if it's necessary.

@gebilaoxiong do you remember why you added the check for (res !== null) at #8036

[hulkyhawk]

@posva when deal with native once event, there is a (res !== null) guard just to make sure the
.self modifier works. see the issure #4846.
but there is no such a problem when the event is component level, the problem should be solved maybe

[posva]

Thanks @hulkyhawk, let's keep this open as the code could potentially be adapted to allow an event listener to return null when .once is added

[gebilaoxiong]

@posva I'm sorry I didn't notice the news;

This is not a broken change，The logic used here is the old one:
https://github.com/vuejs/vue/pull/8036/files#diff-2b06abcad137617975c484105ce8f5a7L26-L28

It looks like a signal value, But I can not find the test-case;