Open Radar

 
Summary:
When using estimatedRowHeight and scrolling quickly past a cell that never gets its heightForRowAtIndexPath called, some cells that are drawn when scrolling back up flicker wildly.

Steps to Reproduce:
1. Create a table with lots of cells. 
2. Provide an `estimatedRowHeight`
3. Implement `tableView:heightForRowAtIndexPath:` and return a different value than `estimatedRowHeight`
4. Call `scrollToRowAtIndexPath:atScrollPosition:animated` to go to the last cell
5. It'll end up halfway down the table thanks to radar 20829131, but that's fine for now.
6. Using your finger, start slowly scrolling back up towards the top of the table.

Expected Results:
The tableview cells should draw smoothly as you scroll back up.

Actual Results:
1. After scrolling 10-15 cells the cells all suddenly jump and scroll much faster than they should be.
2. Things settle down and are normal again until you scroll up another 10 or so cells...

Regression:
Describe circumstances where the problem occurs or does not occur, such as software versions and/or hardware configurations.

Notes:
Manually scrolling fast enough to skip a cell causes this too, but the scrollToRowAtIndexPath method here illustrates it easiest. The wider the difference between estRowHeight and actualHeight, the worse this gets — it is noticeable even with 5pt differences though.