Created attachment 341739 [details]
Image demonstrating bug.
To reproduce, create a contentEditable div with overflowing content and overflow: scroll. Here's a pre-made fiddle for you convenience: https://jsfiddle.net/vyyu0rwc/3/
Click on the content so a caret shows and flashes; now scroll the content (with the caret visible) and notice how the caret position is fixed in place before it disappears, instead of scrolling along with the content. Notice in the attachment (it's pretty bad screenshot), how the caret can appear between lines when scrolling the content due to this bug.