Here it is: https://medium.com/design/system-shock-6b1dc6d6596f#.eqpw885oz

Really only affects Chrome honestly, and they're looking into it. https://bugs.chromium.org/p/chromium/issues/detail?id=627609

We're not using relative units in any place save for our Markdown files (to make our rendered Markdown scale easily for mobile, issue comments, and readmes/blobs).

The few cases of misaligned icons are being fixed as bugs. Overall, we haven't seen too much funkiness across the browsers as the main system fonts are pretty similar size-wise.

Agreed about Roboto btw! Our team is talking about it—we'll probably stick to Roboto or something for headings, but we'll likely remove it for the body text and go to this new font stack.

We feature flag just about every change we do for ease of implementation. It gets our code into production faster for us to test and makes for a smoother release thanks to our internal tools. When we're ready to ship, we just select what percentage to enable that feature flag for—1, 5, 10, 25, 50, 75, or 100%. Goal is to not make the site explode for our users :).

For a font change like this, that wasn't the major concern, but rolling it out incrementally did give me a safety blanket of sorts. In other changes, where we might overhaul a view or unveil a new feature, we want to make sure we don't trigger tons of exceptions inadvertently. A slower rollout means it's easier to respond to those things before they get out of hand.

Hope that helps!