A scroll whose first move past the tap slop leaned sideways (a thumb arc) locked as 'swipe' for the whole contact, and SwipeHandler drops a swipe at 50 px of vertical drift, so the gesture did nothing. Past that limit the drag is now a 'y' scroll. Also brings the mobile-handlers fileoverview to the current 500 ms / 50 px limits and notes in the Mobile Guide that a selection pauses the swipe and a wide terminal pans instead.