The goal is to find a solution for the scenario, that a TextFormField gets the Focus in a page, which is in a PageViewController.
As long as there is no PageViewController or TabController surrounding the page the current solutions provided by Collin Jackson (https://gist.github.com/collinjackson/50172e3547e959cba77e2938f2fe5ff5) and Didier Boealens (https://www.didierboelens.com/2018/04/hint-4-ensure-a-textfield-or-textformfield-is-visible-in-the-viewport-when-has-the-focus/) work fine.
The PageViewController setup is based on the sample app from Collin Jackson too: https://gist.github.com/collinjackson/4fddbfa2830ea3ac033e34622f278824
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