Created attachment 416639 [details]
In the attached testcase, the sticky element gets pushed out of view as you scroll down and should not.
In RenderBoxModelObject::computeStickyPositionConstraints(), containingBlock->layoutOverflowRect() returns a rect that is too short, but only when inside flex layout.
Created attachment 416648 [details]
Committed r271053: <https://trac.webkit.org/changeset/271053>
All reviewed patches have been landed. Closing bug and clearing flags on attachment 416648 [details].