I cannot really see a big change in fps, but it seems like there's something. Eugene, Ali, WDYT? Lindsay, could someone on the test team see if they can get better steps here? 

It looks like it isn't so much that the fps is decreasing, but rather that the transform is updating instantly instead of being animated.

A simpler or more reliable test case would help a lot for debugging this.

Example 1 (on a timer):
https://s.codepen.io/ArthurYidi/debug/qJgKBb/bYrdyeWyQOpA

Example 2 (manual dragging):
https://s.codepen.io/ArthurYidi/debug/VEgxRj/RBMOJNmJaRzk

As you switch tabs in the browser, you will see the transition stops working.

I will note that this bug manifests itself to all subsequent new pages and existing pages that use CSS transition.

Fixed example urls:

Example 1:
https://codepen.io/ArthurYidi/full/qJgKBb

Example 2:
https://codepen.io/ArthurYidi/full/VEgxRj

Tested on 71.0.3578.27 beta,  iPhoneX iOS 11.4.1, iPhoneX, iPhone7 iOS 12.1
Issue is reproducible.

followed the steps in comment#0
https://drive.google.com/file/d/1KxJVPXvSeyF3fiVzqCWMOesYnyikxNhv/view

The video starts with Chrome beta
and 0:28 sec I test safari.

Is this problem reproducible in Firefox? Do we know if this bug is a regression?

Issue repro in Firefox 
https://drive.google.com/file/d/15Rmu8FGEzlZM9-YYY0T6HJ4Tr4QLe56O/view

ajuma@, eugenebut@ If its a WKWebview only issue should we file a radar?


shbarezer@ can you confirm your repro case works on Safari?

We should file radar for this. Not sure if it's a hight priority.

Tested on 71.0.3578.31 beta,  iPhoneX iOS 11.4.1, iPhoneX, iPhone7 iOS 12.1
Issue seems to reproduce in safari as well. video added in comment#5 .

It's possible that there are multiple bugs here, but the example where I find it easiest to see brokenness is the example 1 from comment 4 (https://codepen.io/ArthurYidi/full/qJgKBb). As pointed out above, after switching tabs, not only does the transition on that page break, but, bizarrely, so do transitions on all future tabs too.

However, testing with trunk WebKit and locally built Chrome (and getting past a couple other bugs, https://bugs.webkit.org/show_bug.cgi?id=191598 and https://bugs.webkit.org/show_bug.cgi?id=191700), this no longer reproduces. It's fixed by the WebAnimationsCSSIntegration feature in WebKit.

We should revisit this bug when that feature ships to verify that it's fixed.

Hi there, I've run into this same issue and CSS transitions do indeed stop working when changing tabs. Here is my device and browser info in case it helps:

- Chrome version: 71.0.3578.89  Channel: stable
- OS Version: iOS 12.1.2
- Simple reproducible example: https://codepen.io/anon/pen/oJeejX

Do we know when the WebAnimationsCSSIntegration feature mentioned above will land in Chrome? 

We have to wait for Apple to release that feature in WebKit on iOS. They typically update WebKit on iOS a couple times a year (in recent years, that's happened in March and in September).

Thank you for the super quick response @ajuma - your guy's work on Chromium is truly appreciated! I'm hoping Apple will release in March :praying as CSS transitions are such an important part of the Web platform these days.