Created attachment 466390 [details]
Testcase

Flex layout will call into a flex item's firstLineBaseline/lastLineBaseline in order to compute an ascent for the item to use in baseline alignment. If the flex item has a different writing mode from the flex container then it will return std::nullopt because of the check for isWritingModeRoot() and as a result will have a baseline synthesized. In the attached test case (reduced from css-flexbox/alignment/flex-align-baseline-001.html) we should still be able to compute baseline values for the flex items since the block axis for the items is parallel to the alignment context.