Don't reset the scrollbar when the notebook's scrollable output is updated

[amunger]

1. turn on notebook output scrolling
2. create a cell that will stream output for a while

import time
for i in range(100):
    time.sleep(.1)
    print('hello')

3. Try to scroll down in the output while the output is still being written
   🐛 scroll position keeps bouncing back up to the top