Juxtapose inside an iframe does not receive touch events

[karlcow]

Steps to reproduce

1. With Firefox on Android (tablet or mobile)
2. Go to https://www.lesnumeriques.com/camera-surveillance/netgear-arlo-baby-p37105/test.html
3. reach the first occurence of juxtapose
4. Try to slide right or left

Expected
The slider is moving

Actual
The slider is not moving.

The slider has been added through an iframe.

<iframe class="juxtapose" src="https://cdn.knightlab.com/libs/juxtapose/latest/embed/index.html?uid=555b52ae-0da7-11e8-b263-0edaf8f81e27&amp;wmode=opaque" width="100%" height="433" frameborder="0"></iframe>

This was initially reported on webcompat/web-bugs#15381

When accessing directly https://cdn.knightlab.com/libs/juxtapose/latest/embed/index.html?uid=555b52ae-0da7-11e8-b263-0edaf8f81e27&wmode=opaque
on a mobile or tablet with Firefox Android, this is working.

[karlcow]

ok found the issue!
This is related to https://bugzilla.mozilla.org/show_bug.cgi?id=1178763
because on touchstart in Firefox pageX exists, so we get a negative leftPercent, which then in return makes no sense to move the slider.