Snapshotting UIViews with a forced screen update (i.e., calling -[UIView snapshotAfterScreenUpdates:YES]) sufficiently many times somehow breaks all CSS transform transitions in all WKWebViews embedded in the same app, until the app is-relaunched. In the broken state, transitions are either very janky (low frame rate) or just jump directly to their end state with no intermediate frames.
In an app with a complex view hierarchy (e.g., Chrome for iOS), even a couple calls to -[UIView snapshotAfterScreenUpdates:YES] are enough to get into this broken state. In a simple test app, a few hundred calls are needed to make the jankiness obvious. The UIView being snapshotted is independent of WKWebView (neither contained in WKWebView nor an ancestor). The same bug happens when using -[UIView resizableSnapshotViewFromRect:rect afterScreenUpdates:YES withCapInsets:UIEdgeInsetsZero]. In both cases, changing the 'afterScreenUpdates' argument to NO prevents the bug from happening.
This doesn't seem to affect CSS keyframe animations, just CSS transitions.
I've attached a test app to FB9409883.
This appears to be a bug in UIKit. Tracked by the above radar.