PPK:
Instead of the work I was supposed to do I spent about a day and a half on the alpha version of a viewports visualisation app.
It’s already been very useful to me, since figuring out how the viewports actually work is necessary for full understanding. I hope it does the same for you.
It also contains an example of absolute, fixed, and device-fixed position so that you understand the difference.
If you haven’t read up on the Layout Viewport and Visual Viewport, PPK also got you covered with his previously released “A Tale of Two Viewports”.