REGRESSION (r238090): position:fixed sidebar on https://www.w3.org/TR/SVG2/coords.html does not stay fixed

[Simon Fraser (smfr)]

The position:fixed sidebar on https://www.w3.org/TR/SVG2/coords.html doesn't stay fixed; it scrolls, then snaps when you hover.

[Radar WebKit Bug Importer]

<rdar://problem/46429833>

[Simon Fraser (smfr)]

The page is using main thread scrolling (because of fixed backgrounds), and the compositing update pass is failing to update the positions of fixed elements.

[Simon Fraser (smfr)]

Created attachment 356443 [details]
Patch

[Simon Fraser (smfr)]

Created attachment 356448 [details]
Patch

[zalan]

Comment on attachment 356448 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=356448&action=review

> Source/WebCore/rendering/RenderLayerCompositor.cpp:669
> +    bool isPageScroll = !updateRoot || updateRoot == &rootRenderLayer();

auto

[WebKit Commit Bot]

Comment on attachment 356448 [details]
Patch

Clearing flags on attachment: 356448

Committed r238840: <https://trac.webkit.org/changeset/238840>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.

[Ryan Haddad]

The layout test added with this change is frequently failing, as EWS indicated here:
https://webkit-queues.webkit.org/patch/356448/ios-sim-ews
https://webkit-queues.webkit.org/patch/356448/mac-ews
https://webkit-queues.webkit.org/patch/356448/mac-debug-ews

Link to image diff showing the scrollbar appearing in an unexpected location:
https://build.webkit.org/results/Apple%20Sierra%20Release%20WK1%20(Tests)/r238851%20(14458)/compositing/fixed-with-main-thread-scrolling-diffs.html

[Ryan Haddad]

Reverted r238840 for reason:

The layout test added with this change is frequently failing.

Committed r238855: <https://trac.webkit.org/changeset/238855>

[Ryan Haddad]

This was re-landed in https://trac.webkit.org/changeset/238876/webkit

The test is still failing on the bots:
https://build.webkit.org/results/Apple%20Sierra%20Release%20WK1%20(Tests)/r238877%20(14473)/results.html
http://webkit-test-results.webkit.org/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=compositing%2Ffixed-with-main-thread-scrolling.html

[Simon Fraser (smfr)]

Relanded in https://trac.webkit.org/r238876
Attempted to de-flake the test in https://trac.webkit.org/r238883.