The way I see it, these are the main arguments here:
- Pro: More order, everything stays in place, user knows fixed location of all elements
- Contra: User's working area is limited, browser search may not work correctly
- Contra: Elements may be displaced, depending on how it is implemented (is answer section sticky on the right and is always visible or is it fixed on top and disappears on scroll)
- Pro: Working area is completely free, text scanning is enhanced through easy search in browser
I imagine in a scenario with a limited time frame the user's focus is to work through the given text content as clearly and quickly as possible. Because of this I also imagine the browser search might become an often used tool while looking for specific phrases in a long text.
Overall, the 'feeling' of working with the text can quickly become 'limited' in a nested scroll environment, as you have the impression of not having full control over the content, but having to cherrypick it from the container. On the other hand, having it be displayed fully feels more like a long piece of paper lying in front of you, where you make mental remarks of where important phrases may be.
Because of this I feel like the browser stretched scrolling is the better option here. But it is equally important to always have the answer possibilities in mind, which is why the answer container should definitely remain sticky & visible at all times. Example below.
This all is based on the assumption that the content is very important, of course. If it's not, the two options aren't really that dramatically different.