Mobile Safari: Pinch and Zoom draws lines for a drawing task

[eatyourgreens]

Expected behavior

Multi-touch gestures should be ignored by the drawing code, so pinch-and-zoom should enlarge the subject image.

Current behavior

Via a volunteer on the DMZ Cranes project, who I think is contributing on an iPhone:

I already have an issue with the crane project. I always use my fingers to increase the size and magnification of a photo. Yet the option to choose an animal is already set up. This causes marks to be on the photos which then must be erased before attempting a restart. This has occurred each time and has stopped me from even starting. The plus sign does not allow for easy navigation to be certain of species 😢

Steps to replicate

https://www.zooniverse.org/projects/jocham96/dmz-crane-watch/classify

Pinch and zoom, on a touch device, and you should see a line drawn by accident.

Additional information

This should have been fixed by https://github.com/zooniverse/Panoptes-Front-End/pull/5489. When that PR was merged, mobile Safari didn't support pointer events. I think that what might be happening here is that multi-touch pointer events are triggering drawing.