Item views don't handle insert/remove of rows robustly

This one is tough to describe, but here goes.

When an itemview (I've tested both GridView and ListView and found both to have the same bug) recieves a lot of model row inserts/removes, the view starts messing up very, very badly.

Symptoms:

• Rows will become piled on top of each other
• Rows will appear to have empty gaps (probably due to the above)

See the attached testcase for reproducing. Open it, and scroll around (particularly down to the bottom, repeatedly).

Comments on what appears to be going on under the hood to follow.