How to stack frames inside Auto Layout?

Hello @ypt, Thanks for reaching out and welcome to the Figma community!

Without having a look at your actual design file, it’s a bit challenging to provide a definitive help. However, one approach you might consider is applying an ‘Absolute Position’ to the button.

Once the ‘Absolute Position’ is applied, you can then set constraints to control how the button responds when its parent auto layout frame is resized. Just bear in mind that other auto layout options, such as resizing and advanced layout settings, won’t be available for the button.
For more insights, please take a look at this related article from our Help Center here: Explore auto layout properties - Absolute position

I’ve been exploring our community and came across a couple of files that you might find interesting.:

• Auto Layout Tips & Hacks
• Floating Action Button

Feel free to dive in and explore more on your own at: https://www.figma.com/community
​​There’s a wealth of knowledge and inspiration to be found!

I hope this sheds some light on your query! But if I’ve misunderstood anything, please feel free to clarify.
Also, if possible, sharing your design file with us can be really beneficial! This way, our experienced community designers might be able to provide more tailored advice.

Thanks again for reaching out and being part of our community!