fix(event-manager): Firefox shadow dom event handling

[michaelw85]

Return correct original event target for Firefox

Closes #765

[michaelw85]

@bigopon @fkleuver Followup for #755 #758

[fkleuver]

@bigopon @michaelw85 Do we have any particular tests for this? In v2 I did this a little bit differently, starting with the more specific APIs first and falling back to the more generic ones. I seem to recall discussion that there was something wrong about v2's event handling but not sure if it applied to this.
Should this code be changed to be in sync with v2? https://github.com/aurelia/aurelia/blob/master/packages/runtime-html/src/observation/event-manager.ts#L16
Or should it be the other way around you think? @bigopon
Either way I'd prefer if we kept v1 and v2 the same in this particular regard.

[michaelw85]

@fkleuver I tested the fix locally in my project (created a branch with updated dist). The method is private and I did not think any tests would run in Firefox so I did not write a unit test. I could of course quite easily provide a simple component that could be used to verify the fix (if we run tests in Firefox).

On the approach, I would prefer the v2 setup. This seems correct to me, I thought the composedPath is the "new" standardized api for getting the target info. I was thinking about this approach with my change but decided to minimize impact. If @bigopon agrees I can update the PR to match v2

[bigopon]

If path and composedPath are not the same in chrome, then we cannot move composedPath in front of path, unless we have a way to configure it in the event manager

[michaelw85]

path should be the same, apparently this was the initial proposal by Google and this became composedPath(). composedPath is the standard now.

A quick test in my app shows they are identical in this scenario:

[bigopon]

@michaelw85 thanks for checking, If so let's go with @fkleuver 's suggestion and make it behave like v2

[michaelw85]

ok will update.

@fkleuver I just spotted a mistake in the v2 code. event.deepPath()[0] deepPath is an array not a method.
Documentation: https://developer.mozilla.org/en-US/docs/Web/API/Event

Event.deepPath 
An Array of DOM Nodes through which the event has bubbled.

n.v.m. it was wrong in v1 and the docs I quoted. Based on the original change it's a method: https://codereview.chromium.org/2014173002/#ps100001

[michaelw85]

@bigopon @fkleuver Can this PR be reviewed or are you waiting for me to do any additional changes?

[fkleuver]

I think we're good. Thank you!

@EisenbergEffect Unless you see any issue, as far as I'm concerned we can merge/release this. It's now in sync with v2.

[EisenbergEffect]

Thanks everyone! We'll get this out next week.