Yeah, bug 1526040 approximately covers adding some similar optimizations there. It's a substantial amount of work (mostly to reason about all the edge cases, the invariants that need to be maintained, etc); it's on the layout team's radar and is hopefully getting done sometime this year, but I'm not sure precisely when.
Bug 1591366 Comment 10 Edit History
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Yeah, bug 1524056 approximately covers adding some similar optimizations there. It's a substantial amount of work (mostly to reason about all the edge cases, the invariants that need to be maintained, etc); it's on the layout team's radar and is hopefully getting done sometime this year, but I'm not sure precisely when.