When you have an editable overflow:hidden div, as you type past the end of the div, the overflow layer should scroll to reveal the cursor.
Created attachment 6301 [details]
Created attachment 6524 [details]
Comment on attachment 6524 [details]
Looks good, r=me.
Would be nice to break up that very long line that initialized layerBounds, perhaps putting the width/height on a second line.