Opening the sign-in page, the card would settle into place and then, about three and a half seconds later, the whole page would noticeably grow taller for a fraction of a second before staying still. The page holds a placeholder height while it waits for the sign-in service to answer, then adjusts to fit the real content once it arrives — that placeholder was measured before GitHub sign-in was added as a second option a few days ago, so it no longer matched, and every visitor saw a bigger jump than intended.
Re-measured and corrected. The page now settles at close to its final size straight away, with nothing left to visibly resize.