The CSS Regions spec currently states that "If region chain nesting occurs (the contents of a NamedFlow contains regions for a different NamedFlow) then the regionLayoutUpdate event for the nested flow(s) must be dispatched before the regionLayoutUpdate event for the containing flow is dispatched." However, in the current implementation, first the event for the containing flow is dispatched and then the event for the nested flow.
Created attachment 161460 [details] Ref test highlighting the problem
Created attachment 161711 [details] Ref test highlighting the problem Uploaded new version that doesn't throw JavaScript exceptions in expected file.
The spec has changed to trigger the events at the end of the visual formatting steps, which run outside-in: http://dev.w3.org/csswg/css-regions/#named-flows-layout So the order is correct at this time. We should add a test for this.
Still repros on today's nightly (r151543)
Since the spec doesn't make any specification of event timing for nested regions, this should be closed as soon as we remove the regionlayoutupdate event.
The spec does say how events in nested regions should trigger. The implementation matches the spec at the moment.
CSS Regions were removed in Bug 174978.