On desktop at least, there is a very subtle vertical line coming out of the top of the avatar image of the linked tweet. When you start to scroll up, you'll see that this line is part of the "thread" indicator that links all the posts above it. On my screen, it's about the same as the "x-height" (lower-case letter size) of the username font, at the thickness of the UI grid lines.

It would of course be much better to have the previous tweet partially on-screen, possibly under a small gradient.