Via Ali, we need to set the flattening bit on more cases in spv2:
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cc's PropertyTreeBuilder currently infers that every render surface flattens, so sets flattening bits in the transform tree accordingly. Even after surface knowledge is removed from the transform tree, we'll still need flattening to be correctly represented there (in order to compute correct screen-space transforms).

It looks like blink::PaintPropertyTreeBuilder uses ComputedStyle::preserve3D to set flattening bits (just like CompositedLayerMapping does), but this doesn't currently cover all the things that can cause render surfaces to be created. Looking at the definition of ComputedStyle::usedTransformStyle3D, the flattening reasons that are handled are overflow clipping, filters, reflections, and non-1 opacity. But css clips (these create render surfaces when they’re not axis-aligned wrt the current target), masks, mix-blend-mode, and roots of isolated groups (that is, PaintLayer::shouldIsolateCompositedDescendants) aren't considered.
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