I'll run some tests and try to reproduce the issue.
I ran some tests and could not work around this limitation using regular render.png or render.jpg endpoints.
My recommendation is that your try using the execute endpoint. Programatically you will have access to the page elements and you will be able to not only read elements attributes (like sizes), but also change them.
With the hight at hand you can then resize the viewport to fit your needs: https://splash.readthedocs.io/en/stable/scripting-ref.html#splash-set-viewport-size
Thanks for your help so far! Do you have any resource on hand how I could archive to capture the full height of the website with your proposed solution (the execution endpoint)?
I can try to make a sample script. I'll update you as soon as I have a POC.
I want to render the full height of my page, but if the site contains an inline container it does not work.