Swipes starting at edges (x or y === 0) are disregarded

[merrywhether]

Describe the bug
Swipes originating from coordinates with x = 0 (left edge) or y = 0 (top edge) will not be captured. I only found this by writing tests against my usage of this library as I believe this behavior is really hard to replicate in real world use cases. It's caused by a improper falsy check on this line:
https://github.com/FormidableLabs/react-swipeable/blob/master/src/index.js#L73

I think it should be:

if (state.xy[0] === undefined || state.xy[1] === undefined || ...

Steps or Sandbox to reproduce
In my tests I have

    act(() => {
      // swipe down
      document.dispatchEvent(
        new TouchEvent('touchstart', {
          touches: [
            {
              clientX: 1,
              clientY: 1,
            } as any,
          ],
        }),
      );
      document.dispatchEvent(
        new TouchEvent('touchmove', {
          touches: [
            {
              clientX: 1,
              clientY: 20,
            } as any,
          ],
        }),
      );
      document.dispatchEvent(
        new TouchEvent('touchend', {
          touches: [
            {
              clientX: 1,
              clientY: 20,
            } as any,
          ],
        }),
      );
    });
    expectOnlyHandlerCalled(initialProps.inputHandler, 'onSwipe');
    expect(initialProps.inputHandler.onSwipe).toHaveBeenCalledWith('Down');

As-is, the test passes. But if I change all of the clientX values to 0 (or the initial clientY value to 0), the test fails because no swipe was detected, with the code bailing out in onMove at that if-check.

Expected behavior
Since the screen is a 0-based grid, it seems like touches starting on the edges should be handled?

[hartzis]

@merrywhether nice find. This has existed since this libraries inception.

You're totally right that the actual likelihood that a real user has somehow started a swipe at 0 is extremely low.

I do believe your investigation is correct for where the bug is happening.

react-swipeable/src/index.js

Line 73 in 939ee9f

if (!state.xy[0] || !state.xy[1] || (event.touches && event.touches.length > 1)) {

We also need to update the initialState.xy here to be [].

react-swipeable/src/index.js

Lines 12 to 13 in 939ee9f

	const initialState = {
	xy: [0, 0],

Please feel free to create a PR if you'd like. If not I may be able to get around to this soonish, but my current priorities are working on v6

[hartzis]

Coming in v6