fix(ListViewObserver): support SliverPrototypeExtentList

[LinXunFeng]

Related Issues

• Related to [Feature request] Support for ReorderableListView #158

Description

ReorderableListView is a CustomScrollView with a SliverReorderableList inside, which builds one of the following slivers depending on its parameters:

Parameter	RenderSliver	Before
(default)	RenderSliverList	✅
itemExtent	RenderSliverFixedExtentList	✅
itemExtentBuilder	RenderSliverVariedExtentList	✅
prototypeItem	_RenderSliverPrototypeExtentList	❌

So ListViewObserver already works with ReorderableListView, except when prototypeItem is used. The same applies to ListView(prototypeItem:).

Root cause. ListViewObserver.isSupportRenderSliverType only accepts RenderSliverList, RenderSliverFixedExtentList, and RenderSliverVariedExtentList (matched by its runtimeType string). The private _RenderSliverPrototypeExtentList is none of them, so the sliver context is never found, and both observation and jumpTo/animateTo silently do nothing.

Fix. Check the public abstract base class RenderSliverFixedExtentBoxAdaptor instead, which all three fixed-extent slivers extend:

return obj is RenderSliverList || obj is RenderSliverFixedExtentBoxAdaptor;

This also removes the runtimeType.toString() comparison, which may not match under --obfuscate since class names are obfuscated.

Compatibility

• RenderSliverFixedExtentBoxAdaptor already exists in Flutter 3.7.0 (the minimum supported version), and _RenderSliverPrototypeExtentList extends it there. RenderSliverVariedExtentList extends it since it was added in 3.16.0. Verified against the 3.7.0 and 3.16.0 sources.
• No public API changes.
• Behaviour change for slivers that were previously ignored:
  • They are now picked up by the automatic sliver lookup, so they show up in onObserveAll, and may become the first sliver used by onObserve and jumpTo without sliverContext.
  • SliverViewObserver.extendedHandleObserve is no longer called for them, because the built-in list handling takes over.
  • customTargetRenderSliverType, sliverListContexts, and customHandleObserve still take precedence.

Example

Add ListView - Reorderable demo: drag to reorder, then jump to the item whose value is 50 via items.indexOf. A switch in the app bar toggles prototypeItem, and calls observerController.reattach() because the sliver type changes.

Tests

• Scroll to index > Fixed height with prototypeItem (ListView)
• Scroll to index > ReorderableListView > Dynamic height
• Scroll to index > ReorderableListView > Fixed height with prototypeItem

Both prototypeItem cases fail without the fix.

• flutter test - 86 passed
• flutter analyze - no issues in lib/, test/ and example/
• dart format --set-exit-if-changed lib/ test/
• Run the example on the iOS simulator (Flutter 3.47.1, iPhone 17 Pro Max, iOS 26.5)
• Build and run a minimal ReorderableListView app depending on this branch with Flutter 3.7.0 on the same simulator