Since updating to 3.5.1, I can no longer get single line breaks in the WYSIWYG editor. I used to be able to shift-return for a line break and return for a paragraph break. But now all lines have double spacing. Hitting "delete" at the beginning of the line does not help.
I can of course fix this by switching out of WYSWYG mode, but this makes no sense to me. I tried replacing the WP-ADMIN folder with a fresh copy, no change.