IMHO, IndexedStack says: https://api.flutter.dev/flutter/widgets/IndexedStack-class.html

The stack is always as big as the largest child.

But the offstage property, which IMHO is quite related to Offstage, says: https://api.flutter.dev/flutter/widgets/Offstage-class.html

A widget that lays the child out as if it was in the tree, but ... without taking any room in the parent.

So, I am worried whether making IndexedStack's inactive children offstage will violate its definition?

Thanks for the feedback!

But the offstage property, which IMHO is quite related to Offstage

On the other hand, the documentation of debugVisitOnstageChildren explicitly mentions Overlay as well:

Classes like Offstage and Overlay override this method to hide their children.

To me, that comment left the impression that using an Offstage widget isn't a requirement for an element being considered offstage. That documentation comment further says:

For example, when you instruct the test framework to tap on a widget, by default the finder will look for onstage elements and ignore the offstage ones.

I think this may be a reasonable behavior for IndexedStacks as well. Do you think this might be worth an opt-in flag on IndexedStack instead?

Aha that sounds pretty reasonable to me. So Flutter's "offstage" seems to at least has two meanings, one narrow and one broad.

@Piinks This sounds like something that should definitely be clarified in the docs here if that is the case. It sounds like there are a couple of opportunities listed in the comments here where the docs are not very helpful or clear.

#111524 trys to explain a little bit :)

This did break some customer tests that expect the current behavior.

@simolus3 is there another way we could achieve this? Or a non breaking way we could rework this?

Check out our breaking change policy, and let me know what your thoughts are if you would like to proceed with this change. Thanks!

Or a non breaking way we could rework this?

In c2dd76d, I have made this behavior opt-in. An IndexedStack will continue to behave like it did unless the hiddenChildrenAreOnstage field is disabled.

I chatted with @goderbauer about this recently, and I think this is something we should add to #24722
While your original proposal makes more sense, it is too breaking to change the API as it currently is. I don't know that the opt in flag approach is very beneficial here. I really appreciate you looking into this. I will add this to the list in the linked issue. At this time, I don't think we can accept this change.
Thank you for contributing!

Thanks for the explanation and for taking a look!

Today I realize Flutter is inconsistent here: RenderIndexedStack.debugDescribeChildren says those children are offstage. Thus, when users printing the debug renderobject tree, it will be very surprising - because it is marked as offstage but it is not matched by a offstage finder!

  @override
  List<DiagnosticsNode> debugDescribeChildren() {
    final List<DiagnosticsNode> children = <DiagnosticsNode>[];
    int i = 0;
    RenderObject? child = firstChild;
    while (child != null) {
      children.add(child.toDiagnosticsNode(
        name: 'child ${i + 1}',
        style: i != index ? DiagnosticsTreeStyle.offstage : null,
      ));
      child = (child.parentData! as StackParentData).nextSibling;
      i += 1;
    }
    return children;