In some cases we get a gecko display list that looks like this:
WrapList with asr(<A>, <B>)
Item with asr (<B>) and clipchain(<something> [A])
In this case, we would initialize the WebRenderLayerScrollData for the
nested item using a stop-at ancestor of A (because that was the leafmost
ASR from the containing WrapList) but the item itself has an ASR of B,
which is an ancestor of A. So when walking up from B we'd never hit the
stop-at ancestor, and so we'd end up duplicating metrics from the
containing WRLSD onto the nested WRLSD. This generated an assertion
failure in the APZ code.
This patch detects this scenario and skips adding metrics on the nested
WRLSD. This produces an APZ tree equivalent to what the non-WebRender
path would produce.
MozReview-Commit-ID: 8eo6pzXXKBd
--HG--
extra : rebase_source : 0581c54c4d9fa6ca08249e42b306c7155022bec7
11b43679d8