Add direction-specific pan values to touch-action?

There was a great discussion here about a limitation of touch action:
https://bugs.webkit.org/show_bug.cgi?id=133112#c11.  Basically we agreed
that adding values like 'pan-left' and 'pan-right' (in addition to just
'pan-x') would enable some additional scenarios which Ben Poulain (Safari
input engineer) felt were important.  In particular, it would enable an
event-driven slider to switch to scrolling its container after reaching its
own extent.

I agree with this position.  It's still not a perfect solution to the
problem (I think ultimately you'll need some sort of rich scroll
customization API
<https://docs.google.com/a/chromium.org/document/d/1VnvAqeWFG9JFZfgG5evBqrLGDZYRE5w6G5jEDORekPY/edit#heading=h.kd0gtwwz5bf9>),
but it's a very logical addition and one I'd be happy to add to blink.

What do others think?

Rick