[Webkit-unassigned] [Bug 222381] New: prefers-reduced-motion is not reactive on iOS

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=222381

            Bug ID: 222381
           Summary: prefers-reduced-motion is not reactive on iOS
           Product: WebKit
           Version: Safari 14
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: ldebeasi at gmail.com

Created attachment 421455

  --> https://bugs.webkit.org/attachment.cgi?id=421455&action=review

Code reproduction

When toggling the "Reduce Motion" preference in the Accessibility pane on iOS, the `prefers-reduced-motion` media query is not updated in WebKit.

Steps to reproduce:

1. Open attached code reproduction on an iOS device with the "Reduce Motion" toggle off. This option can be accessed via Settings > Accessibility > Motion > Reduce Motion.
2. Observe that there is a blue square with an animation applied. The animation scales the square as well as fades it out.
3. Turn the "Reduce Motion" toggle on.
4. Observe that the square's animation no longer scales, just fades out.
5. Turn the "Reduce Motion" toggle off.
6. Observe that the square's animation does not begin scaling again.


Expected Behavior:

I would expect that after turning "Reduce Motion" off in step 5 the scaling would start again in the square's animation.



Actual Behavior: 

The scaling does not start again.



Other Info:

- I am able to reproduce this on iOS 14.4 as well as the iOS 14.5 developer beta.
- This works as expected on Safari and Chrome for macOS.

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