RetainableScrollController

[SimonWang9610]

• support RetainableScrollController to avoid scrolling up/down when inserting new items into the top of a ListView

The problem behaves like the below:

not_retain.mp4

The reason that does not adjust the scroll offset by adding jumpTo as a post-frame callback is that the painting may flicker: like:

retain_post_frame.mp4

By adjusting the ScrollPosition.pixels during the layout phase in RetainableScrollController, we could avoid flickering during painting, the final result is like this:

retain.mp4

[SimonWang9610]

support RetainableScrollController