If you select a different context than the top-level one and use :screenshot, the resulting screenshot would be taken inside that context.
This might be confusing for the user? especially when using --fullpage.

However, note that it might be wanted in some case, like when taking a screenshot of a codepen demo (which runs in a remote frame).

Note that there are cases where we absolutely want to execute in the selected context, like :screenshot --selector, where we need to target the right window to retrieve the element.

Bug 1620253 might help for this, by highlighting which context was used for a given evaluation.
We could also provide an easy way to re-execute the evaluation in a different context.