Right-positioned elements jump to the left when dragged in Safari

Demo: http://jsfiddle.net/3fttdwm4/2/

When the element is first dragged, the element will jump to the left and then back into place as the user continues to drag.

This bug is reproducible in Safari but not Chrome.

Start by opening the linked jsFiddle demo in Safari and Chrome and reproduce the first-drag jump for a right-positioned element. Trace the drag behavior from the demo into the repository to identify the relevant entry point; done means right-positioned elements remain in place while dragging in Safari without regressing Chrome behavior.

Written by the indexing model from the issue text.