Bug 196327 – @supports(background-attachment:fixed) should say false on iOS

• Component: CSS
• Status: NEW

[Simon Fraser (smfr)]

We don't support background-attachment:fixed on iOS, so @supports should reflect this.

[Radar WebKit Bug Importer]

<rdar://problem/95035888>

[Sam Sneddon [:gsnedders]]

Is this primarily just the fact that we should be gating support on FixedBackgroundsPaintRelativeToDocument?

[Sam Sneddon [:gsnedders]]

Hmm, we also seem to set FixedElementsLayoutRelativeToFrame on !IOS_FAMILY

[Simon Fraser (smfr)]

`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.

[Sam Sneddon [:gsnedders]]

So the conclusion having spoken to Simon offline is that yes, all that needs to be done is to gate the support (at the parser level, as we do for other conditional values) on FixedBackgroundsPaintRelativeToDocument.

[Tim Nguyen (:ntim)]

https://searchfox.org/wubkat/rev/015533966bd738b8d2392a8ff5f6c92f63ba1085/Source/WebCore/css/CSSProperties.json#1601

Needs to change to:
```
{
   "value": "fixed",
   "settings-flag": "fixedBackgroundsPaintRelativeToDocument"
}
```

[Tim Nguyen (:ntim)]

(In reply to Tim Nguyen (:ntim) from comment #6)
> https://searchfox.org/wubkat/rev/015533966bd738b8d2392a8ff5f6c92f63ba1085/
> Source/WebCore/css/CSSProperties.json#1601
> 
> Needs to change to:
> ```
> {
>    "value": "fixed",
>    "settings-flag": "fixedBackgroundsPaintRelativeToDocument"
> }
> ```

Actually, that settings flag needs to be flipped to work. Alternatively, we would need a custom parser function

[zak ridouh]

Pull request: https://github.com/WebKit/WebKit/pull/30220