Created attachment 220057 [details]
a simplified reproducing content
With contents using css region, ruby base is not rendered on the second region.
The attached content is a simple content to reproduce this issue.
It has "A" as a base of ruby "a" on the first region
and "B" as a base of ruby "b" on the second region.
"B" is not rendered with this content.
It's reproducible with r161096 built from source code.
(and reproducible with nightly build r161093 also)
RenderFlowThread::objectShouldPaintInFlowRegion() seems to prevent "B" from being rendered with region check by regionInRange().
(I've confirmed that "B" is rendered with temporary modification that makes regionInRange() return true.)
This is because the mapped range of regions of "B" is different from the given region of objectShouldPaintInFlowRegion's argument.
Would you mind testing it again with the latest nightly, it works for me.
Tested with 168728.