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@xvrh This patch looks fantastic, sorry it took us so long to look at it. Would you mind rebasing it and force pushing an update? That will cause the tests to restart, so we can see an up to date status.

@Hixie I rebased the branch.
I'm available to make any other requested changes.

+1 this PR

I've been reluctant to review this PR because it's generally not a good idea to turn a State object into public API, since State has many methods that aren't relevant in a case like this. Usually, we've dealt with this case by creating a class like ExpansionTileController that the developer can pass to a ExpansionTile and that exposes only as much API as is needed.

@HansMuller understood!
I went with the public State approach as it the obvious one used by Navigator, Scaffold etc...

I will try with the ExpansionTileController way, taking inspiration from ScrollController, TabController etc...

I'll close this PR and reopen a new one when I have time to work on the ExpansionTileController idea.

@xvrh - there are have been some additional requests for this feature so I'm going to try and land an ExpansionTileController version shortly. What I'm planning is very similar to what you've done, except that the controller's API will be limited to isExpanded, expand() and collapse():

class MyApp extends StatefulWidget {
  const MyApp({super.key});

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  final ExpansionTileController controller = ExpansionTileController();

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        ExpansionTile(
          controller: controller,
          title: const Text('Collapsible menu'),
          children: const <Widget>[
            Text('Menu'),
          ],
        ),
        ElevatedButton(
          onPressed: () {
            if (controller.isExpanded) {
              controller.collapse();
            } else {
              controller.expand();
            }
          },
          child: const Text('TOGGLE MENU'),
        )
      ],
    );
  }
}

This really is the same thing under the hood, so an ExpansionTileController can only control one ExpansionTile.