• I have searched open and closed issues for duplicates
• I am submitting a bug report for existing functionality that does not work as intended
• This isn't a feature request or a discussion topic

Bug description

Some animated APNG stickers are not animated on Signal for iOS. While digging, I realized that the common thread between faulty stickers is that they contain a palette (i.e. PNG with a tRNS chunck).

Having a palette is very important for animated stickers, as it allows to save a lot of weight, thus allowing stickers with higher resolution/more frames to be uploaded.

Note that APNG stickers with palette do work on Signal on all other platforms.

Here are some examples:

Demo sticker with palette

This sticker is not animated on iOS. ❌
https://signal.art/addstickers/#pack_id=b020b5f29795e931cf11c4450d12ebca&pack_key=f2a19052ed4077ffbe59d7374a6a7ed7e433854abb5b5844f0af5eaf3c7f5ade

% pngcheck sticker_with_palette.png 
OK: sticker_with_palette.png (304x304, 8-bit palette+trns, non-interlaced, 24.4%).
% du -h sticker_with_palette.png 
72K	sticker_with_palette.png

Demo sticker without palette (RGB)

This sticker is animated on iOS. ✔️
https://signal.art/addstickers/#pack_id=f30476e7bab76d9b73c289c97bc6fdb4&pack_key=0bb6189c1c7ab04b56a3aa59a69de5101a4a6b0dec54cc3c880732b7be7546d7

% pngcheck sticker_rgb.png
OK: sticker_rgb.png (300x300, 32-bit RGB+alpha, non-interlaced, 67.4%).
% du -h sticker_rgb.png
116K	sticker_rgb.png

Example pack containing both with and without palette stickers

This pack's only sticker that is animated on iOS is the one without a palette:

https://signal.art/addstickers/#pack_id=c8b7a540d40b85697a6f80491f78a04a&pack_key=75aa857b1bd3d7553320da129f4db97a0c10b6f4fceb9264b22d66f4292b318e

% pngcheck *.png
OK: 0.png  (270x300, 8-bit palette+trns, non-interlaced, 91.5%).
OK: 1.png  (320x300, 8-bit palette+trns, non-interlaced, 90.3%).
OK: 2.png  (334x300, 8-bit palette+trns, non-interlaced, 81.7%).
OK: 3.png  (370x300, 8-bit palette+trns, non-interlaced, -13.6%).
OK: 4.png  (270x300, 32-bit RGB+alpha, non-interlaced, 91.7%).
OK: 5.png  (270x300, 8-bit palette+trns, non-interlaced, 82.1%).
OK: 6.png  (270x300, 8-bit palette+trns, non-interlaced, 84.0%).
OK: 7.png  (300x300, 8-bit palette+trns, non-interlaced, 90.7%).
OK: 8.png  (320x300, 8-bit palette+trns, non-interlaced, -6.7%).
OK: 9.png  (270x300, 8-bit palette+trns, non-interlaced, 91.3%).
OK: 10.png (270x300, 8-bit palette+trns, non-interlaced, 89.9%).
OK: 11.png (280x300, 8-bit palette+trns, non-interlaced, 63.4%).
OK: 12.png (270x300, 8-bit palette+trns, non-interlaced, 76.4%).
OK: 13.png (280x300, 8-bit palette+trns, non-interlaced, 59.6%).
OK: 14.png (270x300, 8-bit palette+trns, non-interlaced, 71.4%).
OK: 15.png (270x300, 8-bit palette+trns, non-interlaced, 89.3%).
OK: 16.png (300x300, 8-bit palette+trns, non-interlaced, 68.3%).
OK: 17.png (270x300, 8-bit palette+trns, non-interlaced, 73.5%).
OK: 18.png (280x300, 8-bit palette+trns, non-interlaced, 59.7%).
OK: 19.png (290x300, 8-bit palette+trns, non-interlaced, 68.0%).
OK: 20.png (270x300, 8-bit palette+trns, non-interlaced, 42.6%).
OK: 21.png (300x300, 8-bit palette+trns, non-interlaced, 69.8%).
OK: 22.png (270x300, 8-bit palette+trns, non-interlaced, 50.6%).
OK: 23.png (270x300, 8-bit palette+trns, non-interlaced, 86.7%).

No errors were detected in 24 of the 24 files tested.

Steps to reproduce

• Download a pack containing stickers with a palette on a iOS device (see example above)

Actual result: animated stickers are not animated

Expected result: animated stickers are animated

Device info

Device: iPhone 11 Pro

iOS version: 14.1

Signal version: 3.21.0.1

Note that I don't own an iOS device, so I can't provide debug log or further information.