Created attachment 183824 [details] Patch

I think that, at least for now, it would make sense to only call updateCompositingLayers with the new enumerant if both !hasOutOfFlowPositionedDescendant and usesCompositedScrolling. If both of these are true, we can argue that a) Since we're using comp-scrolling, the composited region presented by the composited-scrolling element to other non-descendant layers doesn't change during composited scrolling (it's always the entire scroll layer), and b) Since we have no out of flow positioned descendants, the scrolling descendants all move together, so their overlap with respect to each other cannot change. So no descendants nor any non-descendants can have their overlap affected, so it's safe to skip testing.

Created attachment 183986 [details] Patch

Comment on attachment 183986 [details] Patch Attachment 183986 [details] did not pass chromium-ews (chromium-xvfb): Output: http://queues.webkit.org/results/16031658 New failing tests: css3/flexbox/flexbox-overflow-auto.html compositing/geometry/limit-layer-bounds-clipping-ancestor.html fast/body-propagation/overflow/002-xhtml.xhtml compositing/overflow/automatically-opt-into-composited-scrolling.html fast/block/float/overhanging-tall-block.html css2.1/20110323/block-non-replaced-height-016.htm fast/body-propagation/overflow/003.html http/tests/eventsource/eventsource-cors-with-credentials.html compositing/overflow/overflow-scrollbar-layers.html compositing/overflow/overflow-auto-with-touch-toggle.html compositing/layer-creation/scroll-partial-update.html http/tests/inspector/search/search-in-concatenated-script.html css3/flexbox/child-overflow.html fast/body-propagation/overflow/007-declarative.xhtml compositing/geometry/limit-layer-bounds-transformed-overflow.html css3/flexbox/flexbox-baseline.html compositing/overflow/clip-content-under-overflow-controls.html fast/body-propagation/overflow/004-declarative.xhtml fast/body-propagation/overflow/002.html fast/body-propagation/overflow/003-declarative.xhtml compositing/overflow/content-gains-scrollbars.html http/tests/inspector/search/search-in-script.html fast/body-propagation/overflow/004-xhtml.xhtml http/tests/inspector/compiler-source-mapping-debug.html compositing/layer-creation/overflow-scroll-overlap.html compositing/overflow/overflow-scroll.html fast/body-propagation/overflow/004.html fast/body-propagation/overflow/003-xhtml.xhtml fast/body-propagation/overflow/007-xhtml.xhtml http/tests/inspector/resource-tree/resource-tree-reload.html

Created attachment 184006 [details] Patch

Created attachment 185246 [details] Patch

Created attachment 185252 [details] Patch for landing Fixed typo in changelog: changed "on-descendant" to "non-descendant".

Comment on attachment 185252 [details] Patch for landing Clearing flags on attachment: 185252 Committed r141121: <http://trac.webkit.org/changeset/141121>

All reviewed patches have been landed. Closing bug.