See discussion/video here: https://twitter.com/patrick_h_lauke/status/775487139664564225

IIRC from our f2f discussion during https://codereview.chromium.org/2022843002 review, Mac low-level "leave" events have no ids, which makes a sensible solution for multi-pointer case almost impossible.

Interesting. Still the common case by far is a single pointer, we could handle that by remembering the last pointer properties (maybe best in Mac specific code), right?

Patrick also mentions he sees a ghost type=mouse event on pen leave, that's probably why.

"Patrick also mentions he sees a ghost type=mouse event on pen leave, that's probably why."

to clarify, this was on the Windows implementation, not on the Mac. you can see this in https://www.youtube.com/watch?v=YddThyDeW2U during 0:20 - 0.50 (though this may well be something that i'm not doing right in my test page's code https://patrickhlauke.github.io/touch/tracker/multi-touch-tracker-pointer-hud.html)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e97eb52977dd7ed3936227f0d240cfeed865e520

commit e97eb52977dd7ed3936227f0d240cfeed865e520
Author: lanwei <lanwei@chromium.org>
Date: Tue Feb 07 21:59:42 2017

Send pointerleave and pointerenter events for styluses on Mac.

We should send the correct pointerleave and pointerenter events when the stylus leaves or enters
the proximity of the tablet on Mac. We will check isEnteringProximity on NSTabletProximity event
to decide the position of styluses. Now we only consider about a simple case that there is only
one pointer device.

The details of how we send pointer leave and enter events are in the doc:
https://docs.google.com/document/d/1O_unOvamElhB_FANTMFJjjG9OIgCxZNGTVSfJzecaSk/

BUG= 624810 

Review-Url: https://codereview.chromium.org/2373733002
Cr-Commit-Position: refs/heads/master@{#448729}

[modify] https://crrev.com/e97eb52977dd7ed3936227f0d240cfeed865e520/content/browser/renderer_host/input/web_input_event_builders_mac.h
[modify] https://crrev.com/e97eb52977dd7ed3936227f0d240cfeed865e520/content/browser/renderer_host/input/web_input_event_builders_mac.mm
[modify] https://crrev.com/e97eb52977dd7ed3936227f0d240cfeed865e520/content/browser/renderer_host/render_widget_host_view_mac.h
[modify] https://crrev.com/e97eb52977dd7ed3936227f0d240cfeed865e520/content/browser/renderer_host/render_widget_host_view_mac.mm
[modify] https://crrev.com/e97eb52977dd7ed3936227f0d240cfeed865e520/ui/base/cocoa/base_view.mm

Lan, can this issue be closed now?

Yes, thanks for checking.