Add pinch-zoom token to touch-action property?

[RByers]

The touch-action property defines a number of tokens. Edge and Chrome also ship an additional pinch-zoom token (and I believe Firefox plans to as well /cc @staktrace). But it's not clear if this will ever make it into the W3C PointerEvents spec.

I think it's particularly important that we increase the visibility of this property (eg. document it on MDN for accessibility reasons. Many image carousels already use touch-action: pan-y when they probably should be using touch-action: pan-y pinch-zoom in order to avoid disabling zooming for vision impared users.

It'll be a little awkward to monkey patch it in, but is it OK if I submit a PR for this?
/cc @dtapuska @teddink @NavidZ @mustaqahmed

[RByers]

Concretely I believe the grammar should change from:
auto | none | [ [ pan-x | pan-left | pan-right ] || [ pan-y | pan-up | pan-down ] ] | manipulation
To:
auto | none | [ [ pan-x | pan-left | pan-right ] || [ pan-y | pan-up | pan-down ] || pinch-zoom ] | manipulation

I.e. pinch-zoom may optionally be specified along with the existing pan values (or by itself), but not in combination with auto, none or manipulation.

manipulation then becomes a synonym for pan-x pan-y pinch-zoom.

[miketaylr]

Assuming this is something Gecko wants to ship eventually (or should otherwise be convinced to...), +1.

[graouts]

Is it expected that pinch-zoom will make it to a Level 3 spec for Pointer Events at W3C?

[RByers]

No such plans unfortunately since it's out of scope for the W3C PEWG charter. But I believe all PE implementations actually implement it, so it may be required for web compatibility (to avoid creating accessibility issues).

[NavidZ]

There is a test for it that we can add it to wpt as well under something like "de-facto" behaviors but not part of any specification.

[RByers]

Oh yeah, it would be great to upstream our test! It doesn't need to be "defacto" - this IS specified in the compat spec, so you can add it https://github.com/web-platform-tests/wpt/tree/master/compat.