Hi brentons@: Could you please specify the OS and Chrome version? We need more data to pin-point your test case---our test seems to suggest that the default pressure (0.5) is exposed for unreported cases:
https://cs.chromium.org/chromium/src/third_party/WebKit/LayoutTests/fast/events/pointerevents/mouse-pointer-event-properties-expected.txt

Canary v55.0.2861.0 on a MacBook Pro.  I have a Wacom Intuos as my primary cursor and used the Mac's trackpad to find pressure == 1.

Lan: The fix here seems closely related to the low-level pressure plumbing (https://codereview.chromium.org/2022843002). If Mac provides a way to check the availability of pressure data, then we should set force=NaN when not available. See:
https://cs.chromium.org/chromium/src/third_party/WebKit/public/platform/WebPointerProperties.h?dr=C&sq=package:chromium&rcl=1474355843&l=63

 Issue 648811  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d0d3d88e1c94e01fb14ecdb3a11f786859236d5b

commit d0d3d88e1c94e01fb14ecdb3a11f786859236d5b
Author: lanwei <lanwei@chromium.org>
Date: Wed Sep 21 20:56:59 2016

Pressure should not be set when the pointer device doesn't support pressure.

When pointer events have a pointer type of mouse, we should not set their pressure,
the default value is 0.5 according to the spec.

In Cocoa Event doc, the pressure is only set for events with a subtype of NSTabletPointEventSubtype.
https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/EventOverview/EventObjectsTypes/EventObjectsTypes.html#//apple_ref/doc/uid/10000060i-CH4-SW4

BUG= 648375 

Review-Url: https://codereview.chromium.org/2361523002
Cr-Commit-Position: refs/heads/master@{#420161}

[modify] https://crrev.com/d0d3d88e1c94e01fb14ecdb3a11f786859236d5b/content/browser/renderer_host/input/web_input_event_builders_mac.mm

@lanwei, it looks like Apple is also sending force for 3D Touch:

https://developer.apple.com/library/content/releasenotes/General/WhatsNewInSafari/Articles/Safari_10_0.html

Should we send that value as pressure in Chrome PointerEvents where pointerType==mouse and the input device is a trackpad?

Unfortunately we do not have this trackpad to test, if we have we will add the pressure for it. But I guess the pressure will be from somewhere else, I noticed that there is special type of NSevent - 'pressure' will handle this case. Right now, the pressure I get is only for stylus devices 'Pointer Events', whose subtype is NSTabletPointEventSubtype.
https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/EventOverview/EventObjectsTypes/EventObjectsTypes.html#//apple_ref/doc/uid/10000060i-CH4-SW4

Then I suggest you buy one.  =)

We should be discussing this in  https://crbug.com/581066