> Our solution to the z-index issue was to make extensive use of React portals to things lower down the doc tree when they were in focus.

I’m curious why you went with portals? Seems to me you could use the normal render cycle to control order (keyed if you intend to just reorder siblings).

> I don't really care about the GPU acceleration; adding layers to SVGs is an amazing change on its own.

I may be mistaken but in my reading of the article, my understanding is that SVG layers are only about acceleration, and don’t change the stacking semantics of SVG elements. But maybe I missed something.