This is especially important for the new text fields, so that you can drag to select hidden text, and the overflow layer needs to scroll to reveal as you drag.
Created attachment 6303 [details]
<rdar://problem/3695120> dragging to scroll doesn't work with overflow:auto/scroll/overlay areas
Created attachment 7077 [details]
This patch will does leave an outstanding issue:
Moving selection below bottom of overflow area causes selection to jump to end
This prevents autoscroll from working well with vertical scrolling in overflow areas. On the plus side, this won't affect the new text fields.
Another loose end is:
Drag & drop doesn't cause editable overflow areas to autoscroll
Comment on attachment 7077 [details]
I made some minor comments about the style on IRC.