title: Prepend list view items while maintaining scroll view offset in Flutter
body: <p>I'm looking for a way to insert new items into a list view, while maintaining the scroll offset of the user. Basically like a twitter feed after pulling to refresh: the new items get added on top, while the scroll position is maintained. The user can then just scroll up to see the newly added items.</p>
  
  <p>If I just rebuild the list/scroll widget with a couple of new items at the beginning, it -of course- jumps, because the height of the scroll view content increased. Just estimating the height of those new items to correct the jump is not an option, because the content of the new items is variable. 
  Even the AnimatedList widget which provides methods to dynamically insert items at an arbitrary position jumps when inserting at index 0.</p>
  
  <p>Any ideas on how to approach this? Perhaps calculating the height of the new items beforehand using an Offstage widget?</p>