Stack Snippets on SO apparently got some CSS changes (at least), which unfortunately cause part of the snippet body to be clipped/obscured in the snippet editor, at least on Chrome (v51.0.2704.79 on Linux):
In the image above the top and left of the input box are clipped off. Replicated it after clearing cache (just in case).
I've used a smallish window for that picture, but the size of the window doesn't seem to matter.
Same thing happens here on MSE, although sometimes it's only the left side not the top.