We have three places in the tree where we calculate some variation of composition bounds, each one slightly different. If we can't refactor them into one function we should at least put them all in the same place so they stay in sync. The three places ares: -RecordFrameMetrics (nsDisplayList.cpp) calculates the composition bounds -CalculateRootCompositionSize (nsDisplayList.cpp) calculates the composition size for the root scrollable layer for use on another layer -nsLayoutUtils::CalculateCompositionSizeForFrame calculates the composition size for a frame. The first and third should be similar enough to be merged.

Ah, I was looking for this bug but didn't find it. Forward-duping.