This assert is checking that the display list by Gecko does not provide multiple stacking contexts which all have picture caching enabled (the
cache_tiles field in
Picture caching currently only works correctly on a single, top-level, stacking context - which is what this assert is checking for.
It's likely that the code in Gecko which determines whether to enable picture caching on a stacking context is getting confused by the OOP iframe and thinking it is a top level window, or something similar.
I notice that the bug specifically is being reproduced when
false. I can make a change so that the assert only triggers when picture caching is enabled, which will stop the assert firing in this test case. However, the root cause of the bug (that Gecko is seemingly supplying an invalid display list) will need to be resolved before picture caching can be enabled in fission mode.
I'll land a patch today that at least only checks the assert if picture caching is enabled.