Didn’t find a report of this, although I have seen it for a while.
I can reproduce this with FireFox 43 (and earlier) and Chromium 45.
When I click on the “reply to link” to open the popup of the post replied to, the view scrolls ever so slightly which is oddly unsettling. But only now I have noticed that the scroll gap seems to be as large as the padding of that post, and it proved to be right, because setting the following CSS solves the issue:
.embedded-posts.top.topic-body {
padding: 0;
}
Did you (or some framework you use) perhaps miss padding (which is 12+15px by default) in your calculations?
I am also unsure about the doubled border-top placement here. Imo, the upper border should have full width, while he second one should either be small as it is now or not exist at all (in which case some white space could be removed). Static screenshot in this post.
Unfortunatly I can not sign your CLA at this moment due to reasons. You will have to make this -2+1 line change yourself (or task someone else wih it).
I mean, you can quote multiple people but afaik Discourse then does its special quote thing anyway.
Except you mean the other direction, i.e. the expansion downward. Which I didn’t actually test. I suppose I should check the CSS classes on those elements.