Chrome change in TouchEvent feature detection

Hey,
One of the big problems with TouchEvents has been that some websites still
assume that support for the TouchEvent API implies a mobile device without
any mouse support (and so clicking with a mouse will not work).

Chrome has long had a hack to partially work around this where we enable
touch support (including the TouchEvent API) only when we detect the
presence of a touchscreen on startup.  With our work on pointer events and
trying to address complaints of customers who plug in a touchscreen after
startup, we've recently overhauled that logic
<https://bugs.chromium.org/p/chromium/issues/detail?id=644318>.  The
principle now is that we're applying the hack JUST to the legacy APIs used
primarily for feature detection [1].  Sites that follow the guidance
<https://hacks.mozilla.org/2013/04/detecting-touch-its-the-why-not-the-how/>
of always listening to both TouchEvents and MouseEvents using modern
addEventListener style instead of the DOM0 'on' attributes will now work
reliably without being impacted by the web compat hack here.  Sites that
condition to special-case "'ontouchstart' in window" will continue to
behave as before.

IMHO this is a good compromise between platform rationality and the web
compat issues and I'm pretty disappointed I didn't think to try this years
ago ;-).  If this works then rather than rely on the (often buggy and
unhelpful) touchscreen detection, I'd probably attempt to simplify this
<https://bugs.chromium.org/p/chromium/issues/detail?id=392584> by having
the legacy feature detection APIs consistently disabled on desktop and
enabled on mobile (while still being overridable from chrome://flags for
special cases).

If this sticks, I'd like to encourage other browsers with desktop
implementations to give it a try and get it formalized as expected behavior
in the spec.  Thoughts?

Rick

[1] For completeness the set of APIs that are hidden in Chrome when there
is no touchscreen are
<https://cs.chromium.org/search/?q=%22RuntimeEnabled%3DTouchEventFeatureDetection%22&sq=package:chromium&type=cs>
:
 on GlobalEventHandlers (window, Document, HTMLElement, SVGElement):
   ontouchstart, ontouchmove, ontouchend, ontouchcancel
 Document.createTouch
 Document.createTouchList
 document.createEvent("TouchEvent")    (throws)