background-attachment:fixed inside position:fixed shouldn't cause slow scrolling

[Cameron McCormack (:heycam)]

Background images that are background-attachment:fixed don't need to cause slow scrolling when inside position:fixed elements, since they won't move.

[Cameron McCormack (:heycam)]

<rdar://72830073>

[Cameron McCormack (:heycam)]

Created attachment 430175 [details]
Squashed patch with dependencies for EWS

[Cameron McCormack (:heycam)]

Created attachment 430232 [details]
Squashed patch with dependencies for EWS

[Cameron McCormack (:heycam)]

Created attachment 430297 [details]
Patch

[Cameron McCormack (:heycam)]

Comment on attachment 430297 [details]
Patch

Let's fix up this build bustage from dropping the bug 226453 patch.

[Cameron McCormack (:heycam)]

Created attachment 430309 [details]
Patch

[Simon Fraser (smfr)]

Comment on attachment 430309 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=430309&action=review

Seems OK, but it's worrisome that some layout tests that don't have fixed backgrounds have different repaint behavior.

> Source/WebCore/rendering/RenderLayerCompositor.cpp:4920
> +            // scrolling tree propagates DescendantScrollersHaveSynchronousScrolling bits up the
> +            // tree, but shouldUpdateScrollLayerPositionSynchronously looks at the scrolling state
> +            // tree instead.)

Yeah that sucks.

[Cameron McCormack (:heycam)]

(In reply to Simon Fraser (smfr) from comment #7)
> Seems OK, but it's worrisome that some layout tests that don't have fixed
> backgrounds have different repaint behavior.

I'll investigate that before landing.

[Cameron McCormack (:heycam)]

>    bool hasSynchronousScrollingReasons(ScrollingNodeID nodeID) const { return !synchronousScrollingReasons(nodeID); }

It was a missing ! here. 😐

[Cameron McCormack (:heycam)]

Created attachment 430438 [details]
Patch

[EWS]

Committed r278419 (238443@main): <https://commits.webkit.org/238443@main>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 430438 [details].