I'll jump ahead to nsPresContext.cpp:
- Its first call is in
nsPresContext::GetInProcessRootContentDocumentPresContext(). That one's definitely fine as-is since it's in an explicitly "in-process" function.
- Its second call is in
nsPresContext::GetRootPresContext(), which seems to be a rabbit-hole in and of itself, with lots of callers that need to be triaged
- Its third call is in
nsPresContext::NotifyInvalidation, which seems to be about handling paints and invalidation. This one looks fine as-is; I don't imagine paint-invalidations would need or want to cross process boundaries.
Besides those and comment 2, the other GetParentPresContext calls are in these files:
Most of these calls look to be APZ-related, and I'm not familiar enough with our APZ code/invariants to know what to make of those. (And I'm not sure which [if any] of those might be GeckoView-specific, which could put them out of scope for the first major fission release.)