ProseMirror seems to affect new lines when pasting in and when copying out.
Sample text:
a
b
c
d
If I copy the text above from a plaintext editor, and paste it into ProseMirror (e.g. the instance on prosemirror.net) it will remove the double new line (in the middle).
If I then copy the text, and paste it back in the plaintext editor, it will add a double new line after each line, like the following:
a
b
c
d
If I copy the text from a rich text editor, then it will add a new line to the double new line (i.e., so it becomes three new lines in a row). Copying from ProseMirror into a rich text editor seems to treat new lines properly, as expected.