The touch-action spec extension in whatwg now defines pinch-zoom to include multi-finger panning. This is what Edge does.

Here is a demo that works in Edge but not in Chrome: http://mustaqahmed.github.io/web/image-panning.html

I see two possible approaches to fix this:

Option A: Redefine when gesture-pinch-begin fires. Currently GestureProvider fires a GPB when a scale gesture is detected. It can instead fire a GPB whenever a multi-finger interaction is detected. Not sure how breaking this could be for other users of GPB/GPE.

In particular, a single GSB/GBE can now contain multiple GPB/GPE sequences. And we have to be careful when to fire GPEs.

Option B: Let TouchActionFilter locally treat a multi-finger gesture as a pinch-zoom. Instead of using the effect of 'touch-action: pinch-zoom' within a GPB/GPE sequence, use it whenever there is a multifinger gesture. A simpler solution would be to assume that it will be effective from the beginning of a multi-finger gesture till GSE.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/16ce460b6793b9fff0c1d4cc72deab60b921a159

commit 16ce460b6793b9fff0c1d4cc72deab60b921a159
Author: mustaq <mustaq@chromium.org>
Date: Tue Mar 14 16:22:36 2017

Fixed two-finger pan filtering with 'touch-action: pan-x pan-y'.

For touch-action filtering, we treat a two-finger pan like a single-finger
pan. As a result, a 'touch-action: pan-x pan-y' region allows two-finger
panning but disallows pinch-zooming. This looks problematic:
- It is not easy for users to distinguish between two-finger panning vs
  pinching.
- Edge's touch-action filtering treats a two-finger pan like a pinch-zoom
  for a while.
- Popular embedded maps need to distinguish between two-finger vs
  one-finger gestures through touch-action, where a two-finger gestures
  is used to manipulate the map while a one-finger gesture is handled by
  the browser. Here is a mock:
    http://mustaqahmed.github.io/web/image-panning.html

This CL filters a two-finger pan like a pinch-zoom to solve this problem.

In addition to solving the bug, this CL also simplified the touch-action
filtering code around panning/zooming.

BUG= 632525 

Review-Url: https://codereview.chromium.org/2726623002
Cr-Commit-Position: refs/heads/master@{#456722}

[modify] https://crrev.com/16ce460b6793b9fff0c1d4cc72deab60b921a159/content/browser/renderer_host/input/touch_action_filter.cc
[modify] https://crrev.com/16ce460b6793b9fff0c1d4cc72deab60b921a159/content/browser/renderer_host/input/touch_action_filter.h
[modify] https://crrev.com/16ce460b6793b9fff0c1d4cc72deab60b921a159/content/browser/renderer_host/input/touch_action_filter_unittest.cc
[modify] https://crrev.com/16ce460b6793b9fff0c1d4cc72deab60b921a159/content/common/input/synthetic_web_input_event_builders.cc
[modify] https://crrev.com/16ce460b6793b9fff0c1d4cc72deab60b921a159/content/common/input/synthetic_web_input_event_builders.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3ce63d7e6d1431b4866d233b4d2e7ea269ebbd3e

commit 3ce63d7e6d1431b4866d233b4d2e7ea269ebbd3e
Author: qyearsley <qyearsley@chromium.org>
Date: Tue Mar 14 20:07:31 2017

Remove baseline with failure for pointervent_touch-action_two-finger...

BUG= 632525 

Review-Url: https://codereview.chromium.org/2754503002
Cr-Commit-Position: refs/heads/master@{#456802}

[delete] https://crrev.com/e2477101857d4c38dd2eb7fb67312b7fd0022f5e/third_party/WebKit/LayoutTests/external/wpt/pointerevents/compat/pointerevent_touch-action_two-finger_interaction-manual-expected.txt