Because a region establishes a stacking context.
Created attachment 213496 [details] Patch
Regarding the test file, i tested the behavior of browsers with a similar test with opacity instead of -webkit-flow-from. Only the WebKit browsers return 0, the other browsers FF/IE/Opera return auto. Since region elements do not have children and paint the flow thread content, i resorted to this test since we already have tests for regions as stacking contexts with siblings.
An old (related) WebKit bug: https://bugs.webkit.org/show_bug.cgi?id=15562 about computed zIndex.
Comment on attachment 213496 [details] Patch View in context: https://bugs.webkit.org/attachment.cgi?id=213496&action=review > Source/WebCore/rendering/style/RenderStyle.h:892 > + bool hasStyleRegion() const { return !rareNonInheritedData->m_regionThread.isNull(); } Would "isNamedFlowRegion" be more precise?
Comment on attachment 213496 [details] Patch r=me,
Comment on attachment 213496 [details] Patch Clearing flags on attachment: 213496 Committed r157121: <http://trac.webkit.org/changeset/157121>
All reviewed patches have been landed. Closing bug.