[Webkit-unassigned] [Bug 228333] New: [iOS] CSS transform transitions become janky after snapshotting other UIViews

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=228333

            Bug ID: 228333
           Summary: [iOS] CSS transform transitions become janky after
                    snapshotting other UIViews
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Animations
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: ajuma at chromium.org
                CC: dino at apple.com, gambard at chromium.org,
                    graouts at apple.com, simon.fraser at apple.com

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.

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