Kind of like how you can do [in-line links](to link people to a website), allow the user to use the same syntax to create contextual information that appears when the [user mouses over](Similar to alt-text on an image). This way users who know the context won’t have to slog through a tedious wall of text while those who don’t can optionally bring themselves up to speed. For clarity sake in-line context will be a different color to a link, and those on mobile (or desktop) can click it to expand out the contained text as if it was part of the original comment.
EDIT: it might be a good idea to potentially use different syntax so that you can link websites within in-line context.
Sounds like something the collapsible spoiler can do.
The problem with that is that you can’t
have spoilers within the line itself
As each spoiler requires a line break
and it just looks awful, both in the markdown, and especially when rendered as a comment.
Compare the almost seamless implementation in my example to the raw output of this comment.