[Webkit-unassigned] [Bug 196327] @supports(background-attachment:fixed) should say NO on iOS

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=196327

--- Comment #4 from Simon Fraser (smfr) <simon.fraser at apple.com> ---
`FixedBackgroundsPaintRelativeToDocument` is historical artifact from iOS originally not supporting `position:fixed` at all.

`We don't support background-attachment:fixed` is not supported on iOS because we didn't want scrolling to trigger a paint on every frame (and we'd still prefer to avoid it). We also can't paint fixed backgrounds synchronously such that they accurately align with the scroll position, so even if we allow them to paint, they will jiggle.

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