1743735 - TouchEvent.targetTouches list saved on touchstart is empty on touchmove

Status: NEW

(Core :: DOM: UI Events & Focus Handling, defect)

Description

[Ksenia Berezina [:ksenia]]

Attachment: test.html

This was initially reported in https://github.com/webcompat/web-bugs/issues/94490 where it's impossible to "navigate" a canvas game by swiping on https://www.starbucks.co.jp/egift_holiday2021/find_gift_santa/

I've attached a reduced test case to replicate the behaviour, it's reproducible in RDM or a Firefox Android device.

The site is saving a reference to event.targetTouches on touchstart and accessing it on touchmove, however in Firefox at that point the list is empty, which causes an error. This is not happening in Chrome.

Comment 1

[Ksenia Berezina [:ksenia]]

I found another example similar to this on https://patrickhlauke.github.io/touch/touchlist-objects/ (on touch the green circle representing targetTouches becomes visible in Chrome, but not in Firefox).

Comment 2

[Henri Sivonen (:hsivonen)]

From the description, seems to meet the definition of S2. Edgar, please downgrade if this should be S3 instead.

Comment 3

[Hsin-Yi Tsai (she/her) | away [:hsinyi]]

Moving to S3, due to the number of webcompat reports we know about so far, also we think websites could work around it by filtering touches with event's target manually. TouchEvent is legacy, and PointerEvent is encouraged to use. :)

Comment 4

[James Graham [:jgraham]]

As far as I can tell we don't know of any current site breakage due to this issue? The original starbucks site seems to be 404.

Comment 5

[Ksenia Berezina [:ksenia]]

Removing webcompat priority for now since there are no site reports anymore.