Comment by weinzierl
"It was a bit tricky to find a union of features that work in both engines [..]"
I am curious what the constraints are to make this work and in which environments it does? Does it work in PDF viewers outside the browser? Is there documentation what is available in which environment? What is enabled by default, can be switched on or off?
I barely looked at Adobe Reader so not sure about that one, it definitely does not work with this PDF though, likely because it's not compliant in several ways. Besides that I wouldn't be surprised if it supports all the required JS APIs and more, just possibly behind some permission prompts.
It might work in Foxit as I believe it supports some scripting. Most of the other native PDF renderers are more static, as far as I know. In either case, I was most interested in the browser-native engines, as I always thought of them as more "static"/limited.
As for documentation on specific features: to be honest, I just looked at the implementations of PDF.js and PDFium. Both only support a subset of the "standard" API, likely for security reasons. But PDF.js for example allows changing a field's background color (colored pixels!), and PDFium allows modifying their position/bounding box (I tried a high res color display by moving a row vertically as if it's a scanline, but things become quite laggy).