Adds a new ScrollNotificationEnd example that demonstrates how to trigger an auto-scroll based on an individual sliver's `SliverConstraints` and `SliverGeometry`.
Then new example auto-scrolls one special "aligned item" sliver to the top or bottom of the viewport, whenever it's partially visible (because it overlaps the top or bottom of the viewport). This example differs from the existing ScrollEndNotification example because the layout of the to-be aligned sliver is retrieved from its `RenderSliver` via a
GlobalKey. The new example does not rely on all of the list items having the same extent.
The scroll notification events reported for a press-drag-release gesture within a scrollable on a touch screen device begin with a `ScrollStartNotification`, followed by a series of `ScrollUpdateNotifications`, and conclude with a `ScrollEndNotification`. This protocol can be used to defer work until an interactive scroll gesture ends. For example, you might defer updating a scrollable's contents via network requests until the scroll has ended, or you might want to automatically auto-scroll at that time.
In the example that follows the CustomScrollView automatically scrolls so that the last partially visible fixed-height item is completely visible when the scroll gesture ends. Many iOS applications do this kind of thing. It only makes sense to auto-scroll when the user isn't actively dragging the scrollable around.
It's easy enough to do this by reacting to a ScrollEndNotifcation by auto-scrolling to align the last fixed-height list item ([source code](https://gist.github.com/HansMuller/13e2a7adadc9afb3803ba7848b20c410)).
https://github.com/flutter/flutter/assets/1377460/a6e6fc77-6742-4f98-81ba-446536535f73
Dragging the scrollbar thumb in a desktop application is a similar user gesture. Currently it's not possible to defer work or auto-scroll (or whatever) while the scrollable is actively being dragged via the scrollbar thumb because each scrollbar thumb motion is mapped to a scroll start - scroll update - scroll end series of notifications. On a desktop platform, the same code behaves quite differently when the scrollbar thumb is dragged.
https://github.com/flutter/flutter/assets/1377460/2593d8a3-639c-407f-80c1-6e6f67fb8c5f
The stream of scroll-end events triggers auto-scrolling every time the thumb moves. From the user's perspective this feels like a losing struggle.
One can also detect the beginning and end of a touch-drag by listening to the value of a ScrollPosition's `isScrollingNotifier`. This approach suffers from a similar problem: during a scrollbar thumb-drag, the `isScrollingNotifier` value isn't updated at all.
This PR refactors the RawScrollbar implementation to effectively use a ScrollDragController to manage scrolls caused by dragging the scrollbar's thumb. Doing so means that dragging the thumb will produce the same notifications as dragging the scrollable on a touch device.
Now desktop applications can choose to respond to scrollbar thumb drags in the same that they respond to drag scrolling on a touch screen. With the changes included here, the desktop or web version of the app works as expected, whether you're listing to scroll notifications or the scroll position's `isScrollingNotifier`.
https://github.com/flutter/flutter/assets/1377460/67435c40-a866-4735-a19b-e3d68eac8139
This PR also makes the second [ScrollPosition API doc example](https://api.flutter.dev/flutter/widgets/ScrollPosition-class.html#cupertino.ScrollPosition.2) work as expected when used with the DartPad that's part of API doc page.
Desktop applications also see scroll start-update-end notifications due to the mouse wheel. There is no touch screen analog for the mouse wheel, so an application that wanted to enable this kind of auto-scrolling alignment would have to include a heuristic that dealt with the sequence of small scrolls triggered by the mouse wheel. Here's an example of that: [source code](https://gist.github.com/HansMuller/ce5c474a458f5f4bcc07b0d621843165). This version of the app does not auto-align in response to small changes, wether they're triggered by dragging the scrollbar thumb of the mouse wheel.
Related sliver utility PRs: https://github.com/flutter/flutter/pull/143538, https://github.com/flutter/flutter/pull/143196, https://github.com/flutter/flutter/pull/143325.
