Recently, when you hover over a profile picture to get an expanded usercard, the comments move downwards. (If there are no comments, then the "Add a comment" link is the screen element which moves downwards.) This appears to be because of the user details moving from next to the picture to below it instead. Since I don't always keep careful track of where my mouse is, this has caused several jump-scares where elements of the page suddenly shifted without an obvious reason.
Here is a series of pictures to make what I am saying clear. First, the bottom of a question (note this happens for both questions and answers, though, I'm using a question merely for convenience):
Now, when I hover to expand the usercard, the comments move downwards (see the increase in whitespace between Catija's user information and the first comment):
I made the expanded usercard see-through by messing with my browser's CSS (just some
visibility: hidden), and it appears that the cause is the user information moving around:
(This last picture has the profile picture from the expanded usercard in a different position, as well as other minor shifts in layout - that's from the browser window being smaller in order to fit the console.)
Please restore the original, expected behavior, which is that hovering the usercard produces a popup but doesn't move anything else around; the expanded usercard simply overlays what was already there.