https://mxr.mozilla.org/mozilla-central/source/layout/base/nsLayoutUtils.cpp#3346 Roc, you added this ifdef'd to MOZ_GFX_OPTIMIZE_MOBILE. Can you add a comment about why its needed?

It's basically saying "on mobile we care more about performance than avoiding a few sampling artifacts".

I don't think mobile is a good indication for that. (Some) modern smart phones have quite substantial horsepower. I we really want to take shortcuts we should gate that on some throughput measurement. I really would like to get consistent rendering on mobile and desktop. At most lets make this a switch (if its a huge perf win, users with low end desktops or netbooks might care too).