Basically, I think the APZ requirement induces a limitation on how aggressively we can collapse WRLSD nodes, which the code to build the WRLSD tree is not respecting.
Bug 1719913 Comment 19 Edit History
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Basically, I think the APZ requirement induces a limitation on how aggressively we can collapse WRLSD nodes (i.e. use the "deferred transform" mechanism to stuff more things into a single WRLSD node), which the code to build the WRLSD tree is not respecting.