Elements in the document flow (flow-into: none) create a composited layer when they have a 3D transform applied to them. When flowing such an element inside a named flow the composited layer isn't preserved/created anymore.
In the attached file, toggle between `flow-into: flow` and `flow-into: none` for the gray div with text - when flow-into is specified, a composited layer is no longer created (this can be inspected via the Debug > Drawing/Compositing flags > Show compositing borders or Debug > Show RenderLayer tree menu entries in Safari/WK nightly)
Created attachment 202983 [details]
Created attachment 203525 [details]
Still repros on today's nightly (r151543)
Fixed in WebKit nighly which was expected after we enabled ACC for elements displayed in regions.