Don't use offsetX/offsetY.

Since coalesced points aren't hit tested and don't have a target.  Fields relative to the target location return the same offset as the clientX/clientY.

We might want to call this out explicitly in the coalesced event spec I guess.
 But if you read: https://drafts.csswg.org/cssom-view/#dom-mouseevent-offsetx then you will likely see that since coalesced events aren't actually dispatched then offsetX ends up being clientX.

I believe we didn't assign these events a target because we didn't want to run through the list at every step of the event dispatch stage.

ie. A bubbling event to a parent will return a different offsetX at each target as it is dispatched to in the event path.

But I agree this will cause a bunch of interop issues like this.

Without offsetX/Y on the coalesced events, I have to hit the DOM to calculate the offsets which is a potentially big performance hit.

The parent event has a target set; I don't see why the children wouldn't have the same target / offset as the parent event.

> I believe we didn't assign these events a target because we didn't want to run through the list at every step of the event dispatch stage.
> ie. A bubbling event to a parent will return a different offsetX at each target as it is dispatched to in the event path.

I don't think that's true.  'currentTarget' changes during bubbling, but 'target' doesn't except for the special case of ShadowDOM retargetting.

> But I agree this will cause a bunch of interop issues like this.

There shouldn't be "interop" issues in that all browsers should behave this way since it's required by the spec for offsetX (assuming that it's actually pageX we return as required by the spec, not clientX).  Perhaps we should add a web-platform-test to validate the behavior though?

But I agree it could cause confusion, especially since we want people to be able to convert/share existing code to use coalesced events.  I filed a spec issue to discuss the design further: https://github.com/w3c/pointerevents/issues/209

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/edf6be53834747cea98abe29b96605a0ca3e1dbd

commit edf6be53834747cea98abe29b96605a0ca3e1dbd
Author: nzolghadr <nzolghadr@chromium.org>
Date: Wed May 24 17:50:42 2017

Set the target lazily for the coalesced events

Set the target for the coalesced events when
they are accessed. This will cause properties
like offsetX/offsetY to be correctly calculated.

BUG= 721485 

Review-Url: https://codereview.chromium.org/2889733002
Cr-Commit-Position: refs/heads/master@{#474347}

[modify] https://crrev.com/edf6be53834747cea98abe29b96605a0ca3e1dbd/third_party/WebKit/LayoutTests/external/wpt/pointerevents/extension/pointerevent_constructor.html
[modify] https://crrev.com/edf6be53834747cea98abe29b96605a0ca3e1dbd/third_party/WebKit/Source/core/events/MouseEvent.h
[modify] https://crrev.com/edf6be53834747cea98abe29b96605a0ca3e1dbd/third_party/WebKit/Source/core/events/PointerEvent.cpp
[modify] https://crrev.com/edf6be53834747cea98abe29b96605a0ca3e1dbd/third_party/WebKit/Source/core/events/PointerEvent.h