[Webkit-unassigned] [Bug 228833] New: iOS 14.5 Performance Regression - Style Invalidation and Painting with hardware accelerated animations

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=228833

            Bug ID: 228833
           Summary: iOS 14.5 Performance Regression - Style Invalidation
                    and Painting with hardware accelerated animations
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Animations
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: ldebeasi at gmail.com
                CC: dino at apple.com, graouts at apple.com

Created attachment 434994

  --> https://bugs.webkit.org/attachment.cgi?id=434994&action=review

Code reproduction

When using Web Animations, WebKit continually invalidates + recalculates style and paints even when using only transform or opacity.

I can reproduce this issue in iOS 14.5+ and cannot reproduce this issue in iOS 14.4 and older which is why I think this is a performance regression.

Steps to reproduce:

1. Open code reproduction on an iPhone running iOS 14.5 or newer. You should see two blue circles translating up the page. This animation will run continuously.
2. Open Dev Tools and take a Timeline recording. You should notice that WebKit is continually painting and invalidating/recalculating styles. You should also notice that both the CPU usage and energy impact are moderate to high.
4. Repeat steps 1-2 on a device running iOS 14.4. You should notice that WebKit does not continually paint and invalidate/recalculate styles. You should also notice that both the CPU usage and energy impact are low.

Expected Behavior:

I would expect that WebKit does not paint and invalidate/recalculate styles on a running animation that can be hardware accelerated.

Actual Behavior:

WebKit paints and invalidates/recalculates styles on a running animation that can be hardware accelerated.

Other Information:

- This has significant performance implications for any website making use of animations in Safari/WebKit as the main thread is constantly performing work. Additionally, this increases the strain on the battery resulting in the battery getting drained faster.
- This does not reproduce in Chrome or Firefox.

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