Issue I opened on the Flutter repo: flutter/flutter#115695

When working with a lengthy Form widget inside a Stepper widget with multiple Steps, only form fields visible are recognized. That leads to wrongfully ignoring methods called on those form fields.
This is especially problematic if, for example, you fill out a field in step 1, go to the last step, fill something out there and call .save(). If many steps are present, the value filled out in the top field will not be saved.

The Material Stepper widget internally uses a ListView to show its Steps. ListView (in contrast to SingleChildScrollView) uses lazy loading for its items, which means that Steps currently not visible in the Stepper are ignored. In case of having FormFields inside the Steps, their methods (like .save()) are not called.

I created this repository to show the bug in a minimal reproducible way.
In form_stepper.dart, a Stepper with 10 Steps and 10 TextFormFields per Step is created.
The _savedFieldsPerStep map is used to showcase the problem: The .save() method of each TextFormField should increment the value for its step number as key. This means, when saving the form, each key of the map should have 10 as value.
However, what you will witness is that saving at the top will leave the bottom steps at 0 saved fields and vice versa.

A way to prevent the faulty lazy-loading mechanism of the Stepper would be to use SingleChildScrollView instead of ListView internally.
However, this would lead to problems with performance, especially for usages of the Stepper where lazy loading is actually desired.
Therefore, I propose to add a bool parameter to the Stepper widget, which allows to disable lazy loading and switch the internal ListView to a SingleChildScrollView with a Column as its child.