I took over a month away from other work I have to do to develop the planeshift crate [1] to solve this exact problem. On the WebRender side, tiling work is advancing quickly as well. OS compositing in WR hasn't landed yet, mostly because work has to be done on the legacy Gecko side to wrangle the Cocoa widgets properly, but it's not true that nobody cares.
[1]: https://github.com/pcwalton/planeshift